[gdb/symtab] Fix Dwarf Error: cannot find DIE

[gdb/symtab] Fix Dwarf Error: cannot find DIE

When loading the debug info package
libLLVM.so.10-10.0.1-lp152.30.4.x86_64.debug from openSUSE Leap 15.2, we
run into a dwarf error:
...
$ gdb -q -batch libLLVM.so.10-10.0.1-lp152.30.4.x86_64.debug
Dwarf Error: Cannot not find DIE at 0x18a936e7 \
  [from module libLLVM.so.10-10.0.1-lp152.30.4.x86_64.debug]
...
The DIE @ 0x18a936e7 does in fact exist, and is part of a CU @ 0x18a23e52.
No error message is printed when using -readnow.

What happens is the following:
- a dwarf2_per_cu_data P is created for the CU.
- a dwarf2_cu A is created for the same CU.
- another dwarf2_cu B is created for the same CU.
- the dwarf2_cu B is set in per_objfile->m_dwarf2_cus, such that
  per_objfile->get_cu (P) returns B.
- P->load_all_dies is set to 1.
- all dies are read into the A->partial_dies htab
- dwarf2_cu A is destroyed.
- we try to find the partial_die for the DIE @ 0x18a936e7 in B->partial_dies.
  We can't find it, but do not try to load all dies, because P->load_all_dies
  is already set to 1.
- an error message is generated.

The question is why we're creating dwarf2_cu A and B for the same CU.

The dwarf2_cu A is created here:
...
 (gdb) bt
 #0  dwarf2_cu::dwarf2_cu (this=0x79a9660, per_cu=0x23c0b30,
     per_objfile=0x1ad01b0) at dwarf2/cu.c:38
 #1  0x0000000000675799 in cutu_reader::cutu_reader (this=0x7fffffffd040,
     this_cu=0x23c0b30, per_objfile=0x1ad01b0, abbrev_table=0x0,
     existing_cu=0x0, skip_partial=false) at dwarf2/read.c:6487
 #2  0x0000000000676eb3 in process_psymtab_comp_unit (this_cu=0x23c0b30,
      per_objfile=0x1ad01b0, want_partial_unit=false,
      pretend_language=language_minimal) at dwarf2/read.c:7028
...

And the dwarf2_cu B is created here:
...
 (gdb) bt
 #0  dwarf2_cu::dwarf2_cu (this=0x885e8c0, per_cu=0x23c0b30,
     per_objfile=0x1ad01b0) at dwarf2/cu.c:38
 #1  0x0000000000675799 in cutu_reader::cutu_reader (this=0x7fffffffcc50,
     this_cu=0x23c0b30, per_objfile=0x1ad01b0, abbrev_table=0x0,
     existing_cu=0x0, skip_partial=false) at dwarf2/read.c:6487
 #2  0x0000000000678118 in load_partial_comp_unit (this_cu=0x23c0b30,
     per_objfile=0x1ad01b0, existing_cu=0x0) at dwarf2/read.c:7436
 #3  0x000000000069721d in find_partial_die (sect_off=(unknown: 0x18a55054),
     offset_in_dwz=0, cu=0x0) at dwarf2/read.c:19391
 #4  0x000000000069755b in partial_die_info::fixup (this=0x9096900,
     cu=0xa6a85f0) at dwarf2/read.c:19512
 #5  0x0000000000697586 in partial_die_info::fixup (this=0x8629bb0,
     cu=0xa6a85f0) at dwarf2/read.c:19516
 #6  0x00000000006787b1 in scan_partial_symbols (first_die=0x8629b40,
     lowpc=0x7fffffffcf58, highpc=0x7fffffffcf50, set_addrmap=0, cu=0x79a9660)
     at dwarf2/read.c:7563
 #7  0x0000000000678878 in scan_partial_symbols (first_die=0x796ebf0,
     lowpc=0x7fffffffcf58, highpc=0x7fffffffcf50, set_addrmap=0, cu=0x79a9660)
     at dwarf2/read.c:7580
 #8  0x0000000000676b82 in process_psymtab_comp_unit_reader
     (reader=0x7fffffffd040, info_ptr=0x7fffc1b3f29b, comp_unit_die=0x6ea90f0,
     pretend_language=language_minimal) at dwarf2/read.c:6954
 #9  0x0000000000676ffd in process_psymtab_comp_unit (this_cu=0x23c0b30,
     per_objfile=0x1ad01b0, want_partial_unit=false,
     pretend_language=language_minimal) at dwarf2/read.c:7057
...

So in frame #9, a cutu_reader is created with dwarf2_cu A.  Then a fixup takes
us to the following CU @ 0x18aa33d6, in frame #5.  And a similar fixup in
frame #4 takes us back to CU @ 0x18a23e52.  At that point, there's no
information available that we're already trying to read that CU, and we end up
creating another cutu_reader with dwarf2_cu B.

It seems that there are two related problems:
- creating two dwarf2_cu's is not optimal
- the unoptimal case is not handled correctly

This patch addresses the last problem, by moving the load_all_dies flag from
dwarf2_per_cu_data to dwarf2_cu, such that it is paired with the partial_dies
field, which ensures that the two can be kept in sync.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-05-27  Tom de Vries  <tdevries@suse.de>

	PR symtab/27898
	* dwarf2/cu.c (dwarf2_cu::dwarf2_cu): Add load_all_dies init.
	* dwarf2/cu.h (dwarf2_cu): Add load_all_dies field.
	* dwarf2/read.c (load_partial_dies, find_partial_die): Update.
	* dwarf2/read.h (dwarf2_per_cu_data::dwarf2_per_cu_data): Remove
	load_all_dies init.
	(dwarf2_per_cu_data): Remove load_all_dies field.