[gdb/symtab] Recognize .gdb_index symbol table with empty entries as empty

When reading a .gdb_index that contains a non-empty symbol table with only
empty entries, gdb doesn't recognize it as empty.

Fix this by recognizing that the constant pool is empty, and then setting the
symbol table to empty.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-08-01  Tom de Vries  <tdevries@suse.de>

	PR symtab/28159
	* dwarf2/read.c (read_gdb_index_from_buffer): Handle symbol table
	filled with empty entries.

gdb/testsuite/ChangeLog:

2021-08-01  Tom de Vries  <tdevries@suse.de>

	PR symtab/28159
	* gdb.dwarf2/dw2-zero-range.exp: Remove kfail.

gdb/dwarf2/read.c

@@ -2777,6 +2777,15 @@ to use the section anyway."),
   ++i;
   map->constant_pool = buffer.slice (metadata[i]);
 
+  if (map->constant_pool.empty () && !map->symbol_table.empty ())
+    {
+      /* An empty constant pool implies that all symbol table entries are
+	 empty.  Make map->symbol_table.empty () == true.  */
+      map->symbol_table
+	= offset_view (gdb::array_view<const gdb_byte> (symbol_table,
+							symbol_table));
+    }
+
   return 1;
 }
 

gdb/testsuite/gdb.dwarf2/dw2-zero-range.exp

@@ -139,16 +139,7 @@ foreach_with_prefix ranges_sect {ranges rnglists} {
 	    }
 	}
 
-	if { $have_complaint } {
-	    pass $test
-	} else {
-	    set index [have_index $lib1]
-	    if { $index == "gdb_index" } {
-		kfail symtab/28159 $test
-	    } else {
-		fail $test
-	    }
-	}
+	gdb_assert { $have_complaint } $test
     }
 
     if { ! $readnow_p } {
@@ -164,16 +155,7 @@ foreach_with_prefix ranges_sect {ranges rnglists} {
 	gdb_load $lib1
 	set test "Zero address complaint - unrelocated - psymtab"
 	set have_complaint [regexp $re.* $gdb_file_cmd_msg]
-	if { $have_complaint } {
-	    pass $test
-	} else {
-	    set index [have_index $lib1]
-	    if { $index == "gdb_index" } {
-		kfail symtab/28159 $test
-	    } else {
-		fail $test
-	    }
-	}
+	gdb_assert { $have_complaint } $test
     }
 
     if { ! $readnow_p } {