[gdb/symtab] Fix fsanitize=address errors for per_cu fields

When building gdb with -fsanitize=thread and gcc 12, and running test-case
gdb.dwarf2/dwz.exp, we run into a data race between:
...
  Read of size 1 at 0x7b200000300d by thread T2:^M
    #0 cutu_reader::cutu_reader(dwarf2_per_cu_data*, dwarf2_per_objfile*, \
    abbrev_table*, dwarf2_cu*, bool, abbrev_cache*) gdb/dwarf2/read.c:6164 \
    (gdb+0x82ec95)^M
...
and:
...
  Previous write of size 1 at 0x7b200000300d by main thread:^M
    #0 prepare_one_comp_unit gdb/dwarf2/read.c:23588 (gdb+0x86f973)^M
...

In other words, between:
...
  if (this_cu->reading_dwo_directly)
...
and:
...
    cu->per_cu->lang = pretend_language;
...

Likewise, we run into a data race between:
...
  Write of size 1 at 0x7b200000300e by thread T4:
    #0 process_psymtab_comp_unit gdb/dwarf2/read.c:6789 (gdb+0x830720)
...
and:
...
  Previous read of size 1 at 0x7b200000300e by main thread:
    #0 cutu_reader::cutu_reader(dwarf2_per_cu_data*, dwarf2_per_objfile*, \
    abbrev_table*, dwarf2_cu*, bool, abbrev_cache*) gdb/dwarf2/read.c:6164 \
    (gdb+0x82edab)
...

In other words, between:
...
      this_cu->unit_type = DW_UT_partial;
...
and:
...
  if (this_cu->reading_dwo_directly)
...

Likewise for the write to addresses_seen in cooked_indexer::check_bounds and a
read from is_dwz in dwarf2_find_containing_comp_unit for test-case
gdb.dwarf2/dw2-dir-file-name.exp and target board cc-with-dwz-m.

The problem is that the written fields are part of the same memory location as
the read fields, so executing a read and write in different threads is
undefined behavour.

Making the written fields separate memory locations fixes it:
...
  struct {
    ENUM_BITFIELD (dwarf_unit_type) unit_type : 8;
  };
  struct {
    ENUM_BITFIELD (language) lang : LANGUAGE_BITS;
  };
  struct {
    bool addresses_seen : 1;
  };
...

This increases the size of struct dwarf2_per_cu_data from 120 to 128 (for -m64).

The set of fields has been established experimentally to be the minimal set to
get rid of this type of -fsanitize=thread errors, but more fields might
require the same treatment.

Looking at the properties of the lang field, unlike dwarf_version it's not
available in the unit header, so it will be set the first time during the
parallel cooked index reading.  The same holds for unit_type, and likewise
for addresses_seen.

Tested on x86_64-linux.

---
 gdb/dwarf2/read.h | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index b7a03933aa5..c4b007d064d 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -163,7 +163,9 @@ struct dwarf2_per_cu_data
 
   /* If addresses have been read for this CU (usually from
      .debug_aranges), then this flag is set.  */
-  bool addresses_seen : 1;
+  struct {
+    bool addresses_seen : 1;
+  };
 
   /* A temporary mark bit used when iterating over all CUs in
      expand_symtabs_matching.  */
@@ -173,11 +175,16 @@ struct dwarf2_per_cu_data
      point in trying to read it again next time.  */
   bool files_read : 1;
 
-  /* The unit type of this CU.  */
-  ENUM_BITFIELD (dwarf_unit_type) unit_type : 8;
+  /* Put this in a struct to ensure a separate memory location.  */
+  struct {
+    /* The unit type of this CU.  */
+    ENUM_BITFIELD (dwarf_unit_type) unit_type : 8;
+  };
 
-  /* The language of this CU.  */
-  ENUM_BITFIELD (language) lang : LANGUAGE_BITS;
+  struct {
+    /* The language of this CU.  */
+    ENUM_BITFIELD (language) lang : LANGUAGE_BITS;
+  };
 
   /* True if this CU has been scanned by the indexer; false if
      not.  */