Write index for dwz -m file

Write index for dwz -m file

PR 24445 ("dwz multifile index not written to index cache") exposed the
fact that we are not doing things right when we generate an index for an
object file that has is linked to a dwz file.  The same happens whether
the index is generated with the intent of populating the index cache or
using the save gdb-index command.

The problem can be observed when running these tests with the
cc-with-dwz-m board:

    FAIL: gdb.base/index-cache.exp: test_cache_enabled_hit: check index-cache stats
    FAIL: gdb.dwarf2/gdb-index.exp: index used
    FAIL: gdb.dwarf2/gdb-index.exp: index used after symbol reloading

When generating the index for such file and inspecting the CU list of the
resulting index (with readelf --debug-dump=gdb_index), we can see something
like:

    CU table:
    [  0] 0x0 - 0xb9
    [  1] 0x0 - 0x44

This is supposed to be a sorted list of the ranges of all CUs in the
file this index represents, so already having some overlap is a red
flag.  It turns out that we save the ranges of CUs coming from both the
main file and the dwz file in the same index.

After digging a little bit, it became quite obvious that the index in
the main file should only list the CUs present in the main file, and a
separate index should be generated for the dwz file, listing the CUs
present in that file.

First, that's what happens if you run dwz on a file that already has a
GDB index embedded.  Second, dwarf2read.c has code to read an index from
a dwz file.  The index in the dwz file is actually required to be
present, if the main file has an index.

So this patch changes write_psymtabs_to_index to generate an index for
the dwz file, if present.  That index only contains a CU list, just like
what the dwz tool does when processing a file that already contains an
index.

Some notes about the implementation:

- The file management (creating a temp file, make sure it's
  close/removed on error - in the right order) is a bit heavy in
  write_psymtabs_to_index, and I needed to add a third file.  I factored
  this pattern in a separate class, index_wip_file.
- It became a bit tedious to keep the call to assert_file_size in
  write_psymtabs_to_index, write_gdbindex would have had to return two
  sizes.  Instead, I moved the calls to assert_file_size where the file
  is written.  The downside is that we lose the filename at this point,
  but it was only used for the very improbable case of ftell failing, so
  I think it's not a problem.
- The actual writing of the index file is factored out to
  write_gdbindex_1, so it can be re-used for both index files.
- While the "save gdb-index" command will now write two .gdb-index
  files, this patch does not update the gdb-add-index.sh script, this
  will come in a later patch.

gdb/ChangeLog:

YYYY-MM-DD  Simon Marchi  <simon.marchi@efficios.com>

	PR gdb/24445
	* dwarf-index-write.h (write_psymtabs_to_index): Add
	dwz_basename parameter.
	* dwarf-index-write.c (write_gdbindex): Move file writing to
	write_gdbindex_1.  Change return type void.
	(assert_file_size): Move up, remove filename parameter.
	(write_gdbindex_1): New function.
	(write_debug_names): Change return type to void, call
	assert_file_size.
	(struct index_wip_file): New struct.
	(write_psymtabs_to_index): Add dwz_basename parameter.  Move
	file logic to index_wip_file.  Write index for dwz file if
	needed.
	(save_gdb_index_command): Pass basename of dwz file, if present.
	* dwarf-index-cache.c (index_cache::store): Obtain and pass
	build-id of dwz file, if present.
	* dwarf2read.c (struct dwz_file): Move to dwarf2read.h.
	(dwarf2_get_dwz_file): Likewise.
	* dwarf2read.h (struct dwz_file): Move from dwarf2read.c.
	(dwarf2_get_dwz_file): Likewise.

gdb/testsuite/ChangeLog:

YYYY-MM-DD  Tom de Vries  <tdevries@suse.de>

	PR gdb/24445
	* gdb.dwarf2/gdb-index.exp (add_gdb_index): Update dwz file with
	generated index.