[PATCH 4/5] [gdb/symtab] Work around fsanitize=address false positive for per_cu->unit_type

[Tom de Vries <tdevries@suse.de>]

When building gdb with -fsanitize=thread and gcc 12, and running test-case
gdb.dwarf2/dwz.exp, 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)
...

Both fields are part of the same bitfield, and writing to one field while
reading from another is not a problem, so this is a false positive.

Fix this by moving the unit_type field out of the bitfield.

Use type unsigned char instead of enum dwarf_unit_type to keep requiring only
8 bits.

The size of struct dwarf2_per_cu_data remains the same (at least for -m64).

Tested on x86_64-linux.
---
 gdb/dwarf2/read.h | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index db300b19621..abce4f83f39 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -100,6 +100,7 @@ struct dwarf2_per_cu_data
 {
   dwarf2_per_cu_data ()
     : lang (language_unknown),
+      unit_type {},
       queued (false),
       is_debug_types (false),
       is_dwz (false),
@@ -109,7 +110,6 @@ struct dwarf2_per_cu_data
       addresses_seen (false),
       mark (false),
       files_read (false),
-      unit_type {},
       scanned (false)
   {
   }
@@ -128,6 +128,10 @@ struct dwarf2_per_cu_data
   /* The language of this CU.  */
   language lang;
 
+  /* The unit type of this CU.  We'd like to use dwarf_unit_type but that
+     requires 'int' storage. */
+  unsigned char unit_type;
+
   /* Flag indicating this compilation unit will be read in before
      any of the current compilation units are processed.  */
   unsigned int queued : 1;
@@ -174,9 +178,6 @@ 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;
-
   /* True if this CU has been scanned by the indexer; false if
      not.  */
   std::atomic<bool> scanned;
-- 
2.35.3