Correctly handle forward DIE references in scanner

The cooked index scanner has special code to handle forward DIE
references.  However, a bug report lead to the discovery that this
code does not work -- the "deferred_entry::spec_offset" field is
written to but never used, i.e., the lookup is done using the wrong
key.

This patch fixes the bug and adds a regression test.

The test in the bug itself used a thread_local variable, which
provoked a failure at runtime.  This test instead uses "maint print
objfiles" and then inspects to ensure that the entry in question has a
parent.  This lets us avoid a clang dependency in the test.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30271

gdb/dwarf2/read.c

@@ -16641,9 +16641,8 @@ cooked_indexer::make_index (cutu_reader *reader)
 
   for (const auto &entry : m_deferred_entries)
     {
-      CORE_ADDR key = form_addr (entry.die_offset, m_per_cu->is_dwz);
+      void *obj = m_die_range_map.find (entry.spec_offset);
-      void *obj = m_die_range_map.find (key);
+      cooked_index_entry *parent = static_cast<cooked_index_entry *> (obj);
-      cooked_index_entry *parent = static_cast <cooked_index_entry *> (obj);
       m_index_storage->add (entry.die_offset, entry.tag, entry.flags,
 			    entry.name, parent, m_per_cu);
     }

gdb/testsuite/gdb.dwarf2/forward-spec.exp

@@ -0,0 +1,102 @@
+# Copyright 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check that the DWARF reader works with a a DW_AT_specification that
+# refers to a later DIE.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+standard_testfile main.c -debug.S
+
+# Set up the DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcfile
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C_plus_plus}
+	    {DW_AT_name     $srcfile}
+	    {DW_AT_comp_dir /tmp}
+
+	} {
+	    declare_labels spec myint
+
+	    # The new indexer has special code to compute the full
+	    # name of an object that uses a specification that appears
+	    # later in the DWARF.
+	    DW_TAG_variable {
+		{DW_AT_specification :$spec}
+		{DW_AT_location {
+		    DW_OP_const1u 23
+		    DW_OP_stack_value
+		} SPECIAL_expr}
+	    }
+
+	    myint: DW_TAG_base_type {
+		{DW_AT_byte_size 4 DW_FORM_sdata}
+		{DW_AT_encoding  @DW_ATE_signed}
+		{DW_AT_name      myint}
+	    }
+
+	    DW_TAG_namespace {
+		{DW_AT_name ns}
+	    } {
+		spec: DW_TAG_variable {
+		    {DW_AT_name v}
+		    {DW_AT_type :$myint}
+		    {DW_AT_declaration 1 DW_FORM_flag_present}
+		}
+	    }
+	}
+    }
+}
+
+if {[prepare_for_testing "failed to prepare" ${testfile} \
+	 [list $srcfile $asm_file] {nodebug}]} {
+    return -1
+}
+
+set in_v 0
+gdb_test_multiple "maint print objfiles" "v has a parent" {
+    -re "^ *\\\[\[0-9\]\\\] *..cooked_index_entry\[^\r\n\]*" {
+	set in_v 0
+	exp_continue
+    }
+    -re "^ *name: *v\[\r\n\]*" {
+	set in_v 1
+	exp_continue
+    }
+    -re "^ *parent: *..cooked_index_entry .. (0|$hex)." {
+	if {$in_v} {
+	    if {$expect_out(1,string) == "0"} {
+		fail $gdb_test_name
+	    } else {
+		pass $gdb_test_name
+	    }
+	    set in_v 0
+	}
+	exp_continue
+    }
+    -re "^\[^\r\n\]*\[\r\n\]+" {
+	exp_continue
+    }
+    -re "$gdb_prompt " {
+	# Done.
+    }
+}