Fix for the complaint observed when symbol reading due to unsupported .debug_names form

Following complaint is observed with the executable compiled with -gdwarf-5
and -gpubnames flags - "During symbol reading: Unsupported .debug_names form
DW_FORM_ref4".  This is the form corresponding to DW_IDX_die_offset attribute.
This patch fixes this complaint.  Tested with clang 10.0.0.  Test case used -

int main()
{
int sum,a,b;
sum = a + b;
return sum;
}

clang -gdwarf-5 -gpubnames test.c -o test.out

gdb -q test.out -ex "set complaints 1" -ex "start"

Reading symbols from test.out...
During symbol reading: Unsupported .debug_names form DW_FORM_ref4 \
  [in module test.out]
Temporary breakpoint 1 at 0x400484
Starting program: test.out
During symbol reading: Unsupported .debug_names form DW_FORM_ref4 \
  [in module test.out]
During symbol reading: Unsupported .debug_names form DW_FORM_ref4 \
  [in module test.out]
During symbol reading: Unsupported .debug_names form DW_FORM_ref4 \
  [in module test.out]

gdb/dwarf2/ChangeLog:

2020-05-08  Nitika Achra  <Nitika.Achra@amd.com>

	PR symtab/25952
	* read.c (dw2_debug_names_iterator::next): Handle DW_FORM_ref*
	and DW_IDX_die_offset.  If there is no compilation unit attribute in
	the index entry, then there is a single CU.  Return the CU at O index
	of compilation unit vector.

gdb/testsuite/ChangeLog:

2020-05-08  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/clang-debug-names.exp: Remove PR25952 kfail.

---
 gdb/dwarf2/read.c                              | 18 ++++++++++++++++++
 gdb/testsuite/gdb.dwarf2/clang-debug-names.exp | 13 +------------
 2 files changed, 19 insertions(+), 12 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index ac208991ff..fc48bb6006 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -5352,6 +5352,18 @@ dw2_debug_names_iterator::next ()
 	  ull = read_unsigned_leb128 (abfd, m_addr, &bytes_read);
 	  m_addr += bytes_read;
 	  break;
+	case DW_FORM_ref4:
+	  ull = read_4_bytes (abfd, m_addr);
+	  m_addr += 4;
+	  break;
+	case DW_FORM_ref8:
+	  ull = read_8_bytes (abfd, m_addr);
+	  m_addr += 8;
+	  break;
+	case DW_FORM_ref_sig8:
+	  ull = read_8_bytes (abfd, m_addr);
+	  m_addr += 8;
+	  break;
 	default:
 	  complaint (_("Unsupported .debug_names form %s [in module %s]"),
 		     dwarf_form_name (attr.form),
@@ -5384,6 +5396,12 @@ dw2_debug_names_iterator::next ()
 	    }
 	  per_cu = &dwarf2_per_objfile->get_tu (ull)->per_cu;
 	  break;
+	case DW_IDX_die_offset:
+	  /* In a per-CU index (as opposed to a per-module index), index
+	     entries without CU attribute implicitly refer to the single CU.  */
+	  if (per_cu == NULL)
+	    per_cu = dwarf2_per_objfile->get_cu (0);
+	  break;
 	case DW_IDX_GNU_internal:
 	  if (!m_map.augmentation_is_gdb)
 	    break;
diff --git a/gdb/testsuite/gdb.dwarf2/clang-debug-names.exp b/gdb/testsuite/gdb.dwarf2/clang-debug-names.exp
index 8bd60401c4..a6e33c1d89 100644
--- a/gdb/testsuite/gdb.dwarf2/clang-debug-names.exp
+++ b/gdb/testsuite/gdb.dwarf2/clang-debug-names.exp
@@ -142,15 +142,4 @@ set pass_re \
     [multi_line \
 	 $cmd \
 	 "type = int \\(\\)"]
-set kfail_re \
-    [multi_line \
-	 $cmd \
-	 "type = <unknown return type> \\(\\)"]
-gdb_test_multiple $cmd "" {
-    -re -wrap $pass_re {
-	pass $gdb_test_name
-    }
-    -re -wrap $kfail_re {
-	kfail symtab/25952 $gdb_test_name
-    }
-}
+gdb_test $cmd $pass_re