gdb/dwarf: store addr/offset/ref_addr sizes in dwarf2_per_cu

gdb/dwarf: store addr/offset/ref_addr sizes in dwarf2_per_cu

New in v2:

 - make the test try with indexes by default
 - using uint8_t instead of unsigned char

In some specific circumstances, it is possible for GDB to read a type
unit from a .dwo file without ever reading in the section of the stub in
the main file.  In that case, calling any of these methods:

  - dwarf2_per_cu::addr_size()
  - dwarf2_per_cu::offset_size()
  - dwarf2_per_cu::ref_addr_size()

will cause a crash, because they will try to read the unit header from
the not-read-in section buffer.  See the previous patch for more
details.

The remaining calls to these methods are in the loc.c and expr.c
files.  That is, in the location and expression machinery.  It is
possible to set things up to cause them to trigger a crash, as shown by
the new test, when running it with the cc-with-gdb-index board:

    $ make check TESTS="gdb.dwarf2/fission-type-unit-locexpr.exp" RUNTESTFLAGS="--target_board=cc-with-gdb-index"
    Running /home/simark/src/binutils-gdb/gdb/testsuite/gdb.dwarf2/fission-type-unit-locexpr.exp ...
    ERROR: GDB process no longer exists

The backtrace at the moment of the crash is:

    #0  0x0000555566968b1f in bfd_getl32 (p=0x78) at /home/simark/src/binutils-gdb/bfd/libbfd.c:846
    #1  0x00005555642e51b7 in read_initial_length (abfd=0x7d1ff1eb0e40, buf=0x78 <error: Cannot access memory at address 0x78>, bytes_read=0x7bfff09daca0, handle_nonstd=true)
        at /home/simark/src/binutils-gdb/gdb/dwarf2/leb.c:92
    #2  0x00005555647ca584 in read_unit_head (header=0x7d0ff1e06c70, info_ptr=0x78 <error: Cannot access memory at address 0x78>, section=0x7c3ff1dea7d0, section_kind=ruh_kind::COMPILE)
        at /home/simark/src/binutils-gdb/gdb/dwarf2/unit-head.c:44
    #3  0x000055556452df18 in dwarf2_per_cu::get_header (this=0x7d0ff1e06c40) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:18531
    #4  0x000055556452e10e in dwarf2_per_cu::addr_size (this=0x7d0ff1e06c40) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:18544
    #5  0x0000555564314ac3 in dwarf2_locexpr_baton_eval (dlbaton=0x7bfff0c9a508, frame=..., addr_stack=0x7bfff0b59150, valp=0x7bfff0c9a430, push_values=..., is_reference=0x7bfff0d33030)
        at /home/simark/src/binutils-gdb/gdb/dwarf2/loc.c:1593
    #6  0x0000555564315bd2 in dwarf2_evaluate_property (prop=0x7bfff0c9a450, initial_frame=..., addr_stack=0x7bfff0b59150, value=0x7bfff0c9a430, push_values=...) at /home/simark/src/binutils-gdb/gdb/dwarf2/loc.c:1668
    #7  0x0000555564a14ee1 in resolve_dynamic_field (field=..., addr_stack=0x7bfff0b59150, frame=...) at /home/simark/src/binutils-gdb/gdb/gdbtypes.c:2758
    #8  0x0000555564a15e24 in resolve_dynamic_struct (type=0x7e0ff1f02550, addr_stack=0x7bfff0b59150, frame=...) at /home/simark/src/binutils-gdb/gdb/gdbtypes.c:2839
    #9  0x0000555564a17061 in resolve_dynamic_type_internal (type=0x7e0ff1f02550, addr_stack=0x7bfff0b59150, frame=..., top_level=true) at /home/simark/src/binutils-gdb/gdb/gdbtypes.c:2972
    #10 0x0000555564a17899 in resolve_dynamic_type (type=0x7e0ff1f02550, valaddr=..., addr=0x4010, in_frame=0x7bfff0d32e60) at /home/simark/src/binutils-gdb/gdb/gdbtypes.c:3019
    #11 0x000055556675fb34 in value_from_contents_and_address (type=0x7e0ff1f02550, valaddr=0x0, address=0x4010, frame=...) at /home/simark/src/binutils-gdb/gdb/value.c:3674
    #12 0x00005555666ce911 in get_value_at (type=0x7e0ff1f02550, addr=0x4010, frame=..., lazy=1) at /home/simark/src/binutils-gdb/gdb/valops.c:992
    #13 0x00005555666ceb89 in value_at_lazy (type=0x7e0ff1f02550, addr=0x4010, frame=...) at /home/simark/src/binutils-gdb/gdb/valops.c:1039
    #14 0x000055556491909f in language_defn::read_var_value (this=0x5555725fce40 <minimal_language_defn>, var=0x7e0ff1f02500, var_block=0x7e0ff1f025d0, frame_param=...)
        at /home/simark/src/binutils-gdb/gdb/findvar.c:504
    #15 0x000055556491961b in read_var_value (var=0x7e0ff1f02500, var_block=0x7e0ff1f025d0, frame=...) at /home/simark/src/binutils-gdb/gdb/findvar.c:518
    #16 0x00005555666d1861 in value_of_variable (var=0x7e0ff1f02500, b=0x7e0ff1f025d0) at /home/simark/src/binutils-gdb/gdb/valops.c:1384
    #17 0x00005555647f7099 in evaluate_var_value (noside=EVAL_NORMAL, blk=0x7e0ff1f025d0, var=0x7e0ff1f02500) at /home/simark/src/binutils-gdb/gdb/eval.c:533
    #18 0x00005555647f740c in expr::var_value_operation::evaluate (this=0x7c2ff1e3b690, expect_type=0x0, exp=0x7c2ff1e3aa00, noside=EVAL_NORMAL) at /home/simark/src/binutils-gdb/gdb/eval.c:559
    #19 0x00005555647f3347 in expression::evaluate (this=0x7c2ff1e3aa00, expect_type=0x0, noside=EVAL_NORMAL) at /home/simark/src/binutils-gdb/gdb/eval.c:109
    #20 0x000055556543ac2f in process_print_command_args (args=0x7fffffffe728 "global_var", print_opts=0x7bfff0be4a30, voidprint=true) at /home/simark/src/binutils-gdb/gdb/printcmd.c:1328
    #21 0x000055556543ae65 in print_command_1 (args=0x7fffffffe728 "global_var", voidprint=1) at /home/simark/src/binutils-gdb/gdb/printcmd.c:1341
    #22 0x000055556543b707 in print_command (exp=0x7fffffffe728 "global_var", from_tty=1) at /home/simark/src/binutils-gdb/gdb/printcmd.c:1408

The problem to solve is: in order to evaluate a location expression, we
need to know some information (the various sizes) found in the unit
header.  In that context, it's not possible to get it from
dwarf2_cu::header, like the previous patch did: at the time the
expression is evaluated, the corresponding dwarf2_cu might have been
freed.  We don't want to re-build a dwarf2_cu just for that, it would be
very inefficient.  We could force reading in the dwarf2_per_cu::section
section (in the main file), but we never needed to read that section
before, so it would be better to avoid reading it unnecessarily.

My initial attempt was to store this information in baton objects
(dwarf2_locexpr_baton & co), so that it can be retrieved when the time
comes to evaluate the expressions.  However, it quickly became obvious
that storing it there would be redundant and wasteful.

I instead opted to store this information directly inside dwarf2_per_cu,
making it easily available when evaluating expressions.  These fields
initially have the value 0, and are set by cutu_reader whenever the
unit is parsed.  The various getters (dwarf2_per_cu::addr_size & al) now
just return these fields.

Doing so allows removing anything related to reading the header from
dwarf2_per_cu, which I think is a nice simplification.  This means that
nothing ever needs to read the header from just a dwarf2_per_cu.

It also happens to shrink the dwarf2_per_cu object size a bit, going
from:

    (top-gdb) p sizeof(dwarf2_per_cu)
    $1 = 176

to

    (top-gdb) p sizeof(dwarf2_per_cu)
    $1 = 120

I placed the new fields at this strange location in dwarf2_per_cu
because there happened to be a nice 3 bytes hole there (on Linux amd64
at least).

The new test set things up as described previously.  Note that the crash
only occurs if using the cc-with-gdb-index board.

Change-Id: I50807a1bbb605f0f92606a9e61c026e3376a4fcf
Approved-By: Andrew Burgess <aburgess@redhat.com>