Use RAII to save and restore scalars

This patch replaces many (but not all) uses of
make_cleanup_restore_integer with a simple RAII-based template class.
It also removes the similar restore_execution_direction cleanup in
favor of this new class.  Subsequent patches will replace other
similar cleanups with this class.

The class is typically instantiated using make_scoped_restore.  This
allows for template argument deduction.

2016-10-21  Tom Tromey  <tom@tromey.com>

	* common/scoped_restore.h: New file.
	* utils.h: Include scoped_restore.h.
	* top.c (execute_command_to_string): Use scoped_restore.
	* python/python.c (python_interactive_command): Use
	scoped_restore.
	(python_command, execute_gdb_command): Likewise.
	* printcmd.c (do_one_display): Use scoped_restore.
	* mi/mi-main.c (exec_continue): Use scoped_restore.
	* mi/mi-cmd-var.c (mi_cmd_var_assign): Use scoped_restore.
	* linux-fork.c (checkpoint_command): Use scoped_restore.
	* infrun.c (restore_execution_direction): Remove.
	(fetch_inferior_event): Use scoped_restore.
	* compile/compile.c (compile_file_command): Use
	scoped_restore.
	(compile_code_command, compile_print_command): Likewise.
	* cli/cli-script.c (execute_user_command): Use
	scoped_restore.
	(while_command, if_command, script_from_file): Likewise.
	* arm-tdep.c (arm_insert_single_step_breakpoint): Use
	scoped_restore.

gdb/common/scoped_restore.h

@@ -0,0 +1,99 @@
+/* scoped_restore, a simple class for saving and restoring a value
+
+   Copyright (C) 2016 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 SCOPED_RESTORE_H
+#define SCOPED_RESTORE_H
+
+/* Base class for scoped_restore_tmpl.  */
+struct scoped_restore_base
+{
+};
+
+/* A convenience typedef.  Users of make_scoped_restore declare the
+   local RAII object as having this type.  */
+typedef const scoped_restore_base &scoped_restore;
+
+/* An RAII-based object that saves a variable's value, and then
+   restores it again when this object is destroyed. */
+template<typename T>
+class scoped_restore_tmpl : public scoped_restore_base
+{
+ public:
+
+  /* Create a new scoped_restore object that saves the current value
+     of *VAR.  *VAR will be restored when this scoped_restore object
+     is destroyed.  */
+  scoped_restore_tmpl (T *var)
+    : m_saved_var (var),
+      m_saved_value (*var)
+  {
+  }
+
+  /* Create a new scoped_restore object that saves the current value
+     of *VAR, and sets *VAR to VALUE.  *VAR will be restored when this
+     scoped_restore object is destroyed.  */
+  scoped_restore_tmpl (T *var, T value)
+    : m_saved_var (var),
+      m_saved_value (*var)
+  {
+    *var = value;
+  }
+
+  scoped_restore_tmpl (const scoped_restore_tmpl<T> &other)
+    : m_saved_var (other.m_saved_var),
+      m_saved_value (other.m_saved_value)
+  {
+    other.m_saved_var = NULL;
+  }
+
+  ~scoped_restore_tmpl ()
+  {
+    if (m_saved_var != NULL)
+      *m_saved_var = m_saved_value;
+  }
+
+ private:
+
+  /* No need for this.  It is intentionally not defined anywhere.  */
+  scoped_restore_tmpl &operator= (const scoped_restore_tmpl &);
+
+  /* The saved variable.  */
+  mutable T *m_saved_var;
+
+  /* The saved value.  */
+  const T m_saved_value;
+};
+
+/* Make a scoped_restore.  This is useful because it lets template
+   argument deduction work.  */
+template<typename T>
+scoped_restore_tmpl<T> make_scoped_restore (T *var)
+{
+  return scoped_restore_tmpl<T> (var);
+}
+
+/* Make a scoped_restore.  This is useful because it lets template
+   argument deduction work.  */
+template<typename T>
+scoped_restore_tmpl<T> make_scoped_restore (T *var, T value)
+{
+  return scoped_restore_tmpl<T> (var, value);
+}
+
+#endif /* SCOPED_RESTORE_H */