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

[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.