gdb/testsuite: Add test for case where gdb_demangle returns NULL

This adds a test for the commit:

  commit 4f180d5396741eb65badba70cf5077b7d48f8641
  Date:   Fri Feb 21 08:19:21 2020 -0700

      Check for null result from gdb_demangle

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/cpp-linkage-name.c: New file.
	* gdb.dwarf2/cpp-linkage-name.exp: New file.

1 files changed, 29 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/cpp-linkage-name.c b/gdb/testsuite/gdb.dwarf2/cpp-linkage-name.c
new file mode 100644
index 0000000..c28dece
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/cpp-linkage-name.c
@@ -0,0 +1,29 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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/>.  */
+
+volatile struct
+{
+  int x;
+  int y;
+} global_var;
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+  return global_var.x + global_var.y;
+}