[gdb/symtab] Fix "file index out of range" complaint

With the test-case included in this commit, we run into this FAIL:
...
(gdb) p var^M
During symbol reading: file index out of range^M
$1 = 0^M
(gdb) FAIL: gdb.dwarf2/dw2-no-code-cu.exp: p var with no complaints
...

This is a regression since commit 6d263fe46e0 ("Avoid bad breakpoints with
--gc-sections"), which contains this change in read_file_scope:
...
-  handle_DW_AT_stmt_list (die, cu, fnd, lowpc);
+  if (lowpc != highpc)
+    handle_DW_AT_stmt_list (die, cu, fnd, lowpc);
...

The change intends to avoid a problem with a check in
lnp_state_machine::check_line_address, but also prevents the file and dir
tables from being read, which causes the complaint.

Fix the FAIL by reducing the scope of the "lowpc != highpc" condition to the
call to dwarf_decode_lines in handle_DW_AT_stmt_list.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29561

gdb/dwarf2/read.c

@@ -9498,8 +9498,8 @@ find_file_and_directory (struct die_info *die, struct dwarf2_cu *cu)
 
 static void
 handle_DW_AT_stmt_list (struct die_info *die, struct dwarf2_cu *cu,
-			const file_and_directory &fnd,
-			CORE_ADDR lowpc) /* ARI: editCase function */
+			const file_and_directory &fnd, CORE_ADDR lowpc,
+			bool have_code) /* ARI: editCase function */
 {
   dwarf2_per_objfile *per_objfile = cu->per_objfile;
   struct attribute *attr;
@@ -9586,7 +9586,12 @@ handle_DW_AT_stmt_list (struct die_info *die, struct dwarf2_cu *cu,
       gdb_assert (die->tag != DW_TAG_partial_unit);
     }
   decode_mapping = (die->tag != DW_TAG_partial_unit);
-  dwarf_decode_lines (cu->line_header, cu, lowpc, decode_mapping);
+  /* The have_code check is here because, if LOWPC and HIGHPC are both 0x0,
+     then there won't be any interesting code in the CU, but a check later on
+     (in lnp_state_machine::check_line_address) will fail to properly exclude
+     an entry that was removed via --gc-sections.  */
+  if (have_code)
+    dwarf_decode_lines (cu->line_header, cu, lowpc, decode_mapping);
 }
 
 /* Process DW_TAG_compile_unit or DW_TAG_partial_unit.  */
@@ -9625,13 +9630,8 @@ read_file_scope (struct die_info *die, struct dwarf2_cu *cu)
 
   /* Decode line number information if present.  We do this before
      processing child DIEs, so that the line header table is available
-     for DW_AT_decl_file.  The PC check is here because, if LOWPC and
-     HIGHPC are both 0x0, then there won't be any interesting code in
-     the CU, but a check later on (in
-     lnp_state_machine::check_line_address) will fail to properly
-     exclude an entry that was removed via --gc-sections.  */
-  if (lowpc != highpc)
-    handle_DW_AT_stmt_list (die, cu, fnd, lowpc);
+     for DW_AT_decl_file.  */
+  handle_DW_AT_stmt_list (die, cu, fnd, lowpc, lowpc != highpc);
 
   /* Process all dies in compilation unit.  */
   if (die->child != NULL)

gdb/testsuite/gdb.dwarf2/dw2-no-code-cu.c

@@ -0,0 +1,18 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 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/>.  */
+
+int var;

gdb/testsuite/gdb.dwarf2/dw2-no-code-cu.exp

@@ -0,0 +1,31 @@
+# Copyright 2022 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/>.
+
+standard_testfile .c main.c
+
+if [prepare_for_testing "failed to prepare" $testfile "$srcfile $srcfile2"] {
+    return -1
+}
+
+set cmd "p var"
+
+set re \
+    [multi_line \
+	 "$cmd" \
+	 "\\\$1 = 0"]
+
+with_complaints 5 {
+    gdb_test $cmd $re "$cmd with no complaints"
+}