[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.

3 files changed, 69 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/decl-before-def-decl.c b/gdb/testsuite/gdb.base/decl-before-def-decl.c
new file mode 100644
index 00000000000..3e4e6d777d9
--- /dev/null
+++ b/gdb/testsuite/gdb.base/decl-before-def-decl.c
@@ -0,0 +1,25 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+extern int a[];
+
+int
+main (void)
+{
+  a[0] = 1;
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/decl-before-def-def.c b/gdb/testsuite/gdb.base/decl-before-def-def.c
new file mode 100644
index 00000000000..4d25248bc27
--- /dev/null
+++ b/gdb/testsuite/gdb.base/decl-before-def-def.c
@@ -0,0 +1,18 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int a[2] = { 1, 2 };
diff --git a/gdb/testsuite/gdb.base/decl-before-def.exp b/gdb/testsuite/gdb.base/decl-before-def.exp
new file mode 100644
index 00000000000..feb2084a82f
--- /dev/null
+++ b/gdb/testsuite/gdb.base/decl-before-def.exp
@@ -0,0 +1,26 @@
+# Copyright 2020 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+standard_testfile decl-before-def-decl.c decl-before-def-def.c
+set sources [list $srcfile $srcfile2]
+
+if {[prepare_for_testing "failed to prepare" $testfile $sources]} {
+    return -1
+}
+
+# This is required due to PR25764.
+gdb_test "maint expand-symtabs"
+
+gdb_test "p a" { = \{1, 2\}}