[gdb/symtab] Handle .gdb_index in ada language mode

When running test-case gdb.base/with.exp with target board cc-with-gdb-index,
we have:
...
(gdb) PASS: gdb.base/with.exp: basics: show language
with language ada -- print g_s^M
'g_s' has unknown type; cast it to its declared type^M
(gdb) FAIL: gdb.base/with.exp: basics: with language ada -- print g_s
...

This is due to this bit in dw2_map_matching_symbols:
...
  if (dwarf2_per_objfile->index_table != nullptr)
    {
      /* Ada currently doesn't support .gdb_index (see PR24713).  We can get
	 here though if the current language is Ada for a non-Ada objfile
	 using GNU index.  As Ada does not look for non-Ada symbols this
	 function should just return.  */
      return;
    }
...

While the reasoning in the comment may be sound from language perspective, it
does introduce an inconsistency in gdb behaviour between:
- having a .gdb_index section, and
- having a .gdb_names section, or a partial symtab, or -readnow.

Fix the inconsistency by completing implementation of
dw2_map_matching_symbols.

Tested on x86_64-linux, both with native and target board
cc-with-debug-index.

gdb/ChangeLog:

2020-05-20  Tom de Vries  <tdevries@suse.de>

	PR symtab/25833
	* dwarf2/read.c (dw2_map_matching_symbols): Handle .gdb_index.

gdb/testsuite/ChangeLog:

2020-05-20  Tom de Vries  <tdevries@suse.de>

	PR symtab/25833
	* gdb.base/with-mf-inc.c: New test.
	* gdb.base/with-mf-main.c: New test.
	* gdb.base/with-mf.exp: New file.

1 files changed, 28 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/with-mf-main.c b/gdb/testsuite/gdb.base/with-mf-main.c
new file mode 100644
index 0000000..b50d98f
--- /dev/null
+++ b/gdb/testsuite/gdb.base/with-mf-main.c
@@ -0,0 +1,28 @@
+/* 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/>.  */
+
+/* Main part of with.c.  */
+
+extern void inc (void);
+
+int
+main ()
+{
+  inc ();
+
+  return 0;
+}