[gdb/symtab] Prefer def over decl (inter-CU case)

When running test-case gdb.threads/tls.exp with target board -readnow, we
have:
...
(gdb) print a_thread_local^M
Cannot find thread-local storage for process 0, executable file tls/tls:^M
Cannot find thread-local variables on this target^M
(gdb) FAIL: gdb.threads/tls.exp: print a_thread_local
...
while with native we have:
...
(gdb) print a_thread_local^M
Cannot read `a_thread_local' without registers^M
(gdb) PASS: gdb.threads/tls.exp: print a_thread_local
...

The difference in behaviour can be explained as follows.  Without -readnow, we
have two a_thread_locals, the def and the decl, each in a different CU:
...
$ gdb -batch outputs/gdb.threads/tls/tls \
    -ex "maint expand-symtabs" \
    -ex "print a_thread_local" \
    -ex "maint print symbols" \
    | grep "a_thread_local;"
Cannot read `a_thread_local' without registers
 int a_thread_local; computed at runtime
 int a_thread_local; unresolved
...
and with -readnow, we have the opposite order:
...
$ gdb -readnow -batch outputs/gdb.threads/tls/tls  \
    -ex "maint expand-symtabs" \
    -ex "print a_thread_local" \
    -ex "maint print symbols" \
    | grep "a_thread_local;"
Cannot find thread-local storage for process 0, executable file tls/tls:
Cannot find thread-local variables on this target
 int a_thread_local; unresolved
 int a_thread_local; computed at runtime
...

Fix the FAIL by preferring the def over the decl (something we already do
intra-CU since the fix for PR24971, commit 93e55f0a03 "[gdb/symtab] Prefer var
def over decl").

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-04-23  Tom de Vries  <tdevries@suse.de>

	PR symtab/25807
	* block.c (best_symbol, better_symbol): Promote to external.
	* block.h (best_symbol, better_symbol): Declare.
	* symtab.c (lookup_symbol_in_objfile_symtabs): Prefer def over
	decl.

gdb/testsuite/ChangeLog:

2020-04-23  Tom de Vries  <tdevries@suse.de>

	* gdb.base/decl-before-def-decl.c: New test.
	* gdb.base/decl-before-def-def.c: New test.
	* gdb.base/decl-before-def.exp: New file.

1 files changed, 27 insertions, 7 deletions

diff --git a/gdb/symtab.c b/gdb/symtab.c
index dc079ef..1eef978 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -2285,6 +2285,8 @@ lookup_symbol_in_objfile_symtabs (struct objfile *objfile,
 			  name, domain_name (domain));
     }
 
+  struct block_symbol other;
+  other.symbol = NULL;
   for (compunit_symtab *cust : objfile->compunits ())
     {
       const struct blockvector *bv;
@@ -2295,18 +2297,36 @@ lookup_symbol_in_objfile_symtabs (struct objfile *objfile,
       block = BLOCKVECTOR_BLOCK (bv, block_index);
       result.symbol = block_lookup_symbol_primary (block, name, domain);
       result.block = block;
-      if (result.symbol != NULL)
+      if (result.symbol == NULL)
+	continue;
+      if (best_symbol (result.symbol, domain))
 	{
-	  if (symbol_lookup_debug > 1)
+	  other = result;
+	  break;
+	}
+      if (symbol_matches_domain (result.symbol->language (),
+				 SYMBOL_DOMAIN (result.symbol), domain))
+	{
+	  struct symbol *better
+	    = better_symbol (other.symbol, result.symbol, domain);
+	  if (better != other.symbol)
 	    {
-	      fprintf_unfiltered (gdb_stdlog, " = %s (block %s)\n",
-				  host_address_to_string (result.symbol),
-				  host_address_to_string (block));
+	      other.symbol = better;
+	      other.block = block;
 	    }
-	  result.symbol = fixup_symbol_section (result.symbol, objfile);
-	  return result;
+	}
+    }
 
+  if (other.symbol != NULL)
+    {
+      if (symbol_lookup_debug > 1)
+	{
+	  fprintf_unfiltered (gdb_stdlog, " = %s (block %s)\n",
+			      host_address_to_string (other.symbol),
+			      host_address_to_string (other.block));
 	}
+      other.symbol = fixup_symbol_section (other.symbol, objfile);
+      return other;
     }
 
   if (symbol_lookup_debug > 1)