[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 tentative fix for PR25700 (
https://sourceware.org/pipermail/gdb-patches/2020-April/167451.html ), 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 tentative 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 tentative 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.

The test-case passes on master, starts failing with the tentative patch for
PR25700, and passes again with this patch.

gdb/ChangeLog:

2020-04-08  Tom de Vries  <tdevries@suse.de>

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

gdb/testsuite/ChangeLog:

2020-04-08  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.

---
 gdb/psymtab.c                              | 8 +++-----
 gdb/testsuite/gdb.dwarf2/imported-unit.exp | 6 ++++++
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/gdb/psymtab.c b/gdb/psymtab.c
index d952f453d9..5e4e7d116f 100644
--- a/gdb/psymtab.c
+++ b/gdb/psymtab.c
@@ -157,17 +157,15 @@ psym_map_symtabs_matching_filename
 
   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;
-
       /* Anonymous psymtabs don't have a file name.  */
       if (pst->anonymous)
 	continue;
 
       if (compare_filenames_for_search (pst->filename, name))
 	{
+	  while (pst->user)
+	    pst = pst->user;
+
 	  if (partial_map_expand_apply (objfile, name, real_path,
 					pst, callback))
 	    return true;
diff --git a/gdb/testsuite/gdb.dwarf2/imported-unit.exp b/gdb/testsuite/gdb.dwarf2/imported-unit.exp
index d7b3d4c539..32a9abf620 100644
--- a/gdb/testsuite/gdb.dwarf2/imported-unit.exp
+++ b/gdb/testsuite/gdb.dwarf2/imported-unit.exp
@@ -186,6 +186,12 @@ if { $psymtabs_p } {
     unsupported $test
 }
 
+gdb_test "l imported_unit.c:1" \
+    "1\timported_unit.c: No such file or directory\."
+
+gdb_test "info source" "\r\nCurrent source file is imported_unit.c\r\n.*" \
+    "info source for imported_unit.c"
+
 # Sanity check
 gdb_test "ptype main" "= int \\(void\\)"