gdbsupport: move xfree into its own file

In the next commit I'd like to reference gdb_unique_ptr within the
common-utils.h file.  However, this requires that I include
gdb_unique_ptr.h, which requires that xfree be defined.

Interestingly, gdb_unique_ptr.h doesn't actually include anything that
defines xfree, but I was finding that when I added a gdb_unique_ptr.h
include to common-utils.h I was getting a dependency cycle; before my
change xfree was defined when gdb_unique_ptr.h was processed, while
after my change it was not, and this made g++ unhappy.

To break this cycle, I propose to move xfree into its own header file,
gdb-xfree.h, which I'll then include into gdb_unique_ptr.h and
common-utils.cc.

gdbsupport/gdb-xfree.h

@@ -0,0 +1,41 @@
+/* Copyright (C) 1986-2021 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+#ifndef GDBSUPPORT_GDB_XFREE_H
+#define GDBSUPPORT_GDB_XFREE_H
+
+#include "gdbsupport/poison.h"
+
+/* GDB uses this instead of 'free', it detects when it is called on an
+   invalid type.  */
+
+template <typename T>
+static void
+xfree (T *ptr)
+{
+  static_assert (IsFreeable<T>::value, "Trying to use xfree with a non-POD \
+data type.  Use operator delete instead.");
+
+  if (ptr != NULL)
+#ifdef GNULIB_NAMESPACE
+    GNULIB_NAMESPACE::free (ptr);	/* ARI: free */
+#else
+    free (ptr);				/* ARI: free */
+#endif
+}
+
+#endif /* GDBSUPPORT_GDB_XFREE_H */