[gdb/symtab] Find filename in shared psymtab

[gdb/symtab] Find filename in shared psymtab

When running test-case gdb.ada/dgopt.exp with target board
unix/-flto/-O0/-flto-partition=none/-ffat-lto-objects and gcc-8, gcc-9 or
gcc-10, and the fix for PR25700, we run into this regression:
...
(gdb) list x.adb:16, 16^M
No source file named x.adb.^M
(gdb) FAIL: gdb.ada/dgopt.exp: list x.adb:16, 16
...

The reason for the failure is that without the fix for PR25700, we
have an unshared psymtab:
...
  { psymtab gdb.ada/dgopt/x.adb ((struct partial_symtab *) $hex)^M
    readin no^M
    fullname (null)^M
    text addresses 0x0 -- 0x0^M
    psymtabs_addrmap_supported yes^M
    globals (none)^M
    statics (none)^M
    dependencies (none)^M
  }^M
...
and a shared psymtab (with user field set):
...
  { psymtab gdb.ada/dgopt/x.adb ((struct partial_symtab *) $hex)^M
    readin no^M
    fullname (null)^M
    text addresses 0x0 -- 0x0^M
    psymtabs_addrmap_supported yes^M
    globals (none)^M
    statics (none)^M
    user <artificial>@0x159a ((struct partial_symtab *) 0x37b57c0)^M
    dependencies (none)^M
  }^M
...

The fix for PR25700 removes the unshared psymtab.

Then when trying to find a psymtab matching x.adb in
psym_map_symtabs_matching_filename, we run into this continue for the shared
psymtab:
...
  for (partial_symtab *pst : require_partial_symbols (objfile, true))
    {
      /* We can skip shared psymtabs here, because any file name will be
        attached to the unshared psymtab.  */
      if (pst->user != NULL)
       continue;
...
and consequently cannot find the file.

Fix this by not skipping the shared symtab in
psym_map_symtabs_matching_filename.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-04-22  Tom de Vries  <tdevries@suse.de>

	PR symtab/25801
	* psymtab.c (psym_map_symtabs_matching_filename): Don't skip shared
	symtabs.

gdb/testsuite/ChangeLog:

2020-04-22  Tom de Vries  <tdevries@suse.de>

	PR symtab/25801
	* gdb.dwarf2/imported-unit.exp: Test that we can get imported_unit.c
	in "info source" output.