Make inferior::detaching a bool, and introduce scoped_restore::release()

I left making inferior::detaching a bool to a separate patch, because
doing that makes a make_cleanup_restore_integer call in
infrun.c:prepare_for_detach no longer compile (passing a 'bool *' when
an 'int *' is expected).  Since we want to get rid of cleanups anyway,
I looked at converting that to a scoped_restore.  However,
prepare_for_detach wants to discard the cleanup on success, and
scoped_restore doesn't have an equivalent for that.  So I added one --
I called it "release()" because it seems like a natural fit in the way
standard components call similarly-spirited methods, and, it's also
what the proposal for a generic scope guard calls it too, AFAICS:

  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4189.pdf

I've added some scoped_guard unit tests, while at it.

gdb/ChangeLog:
2017-04-19  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/scoped_restore-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add scoped_restore-selftests.o.
	* common/scoped_restore.h (scoped_restore_base): Make "class".
	(scoped_restore_base::release): New public method.
	(scoped_restore_base::scoped_restore_base): New protected ctor.
	(scoped_restore_base::m_saved_var): New protected field.
	(scoped_restore_tmpl::scoped_restore_tmpl(T*)): Initialize the
	scoped_restore_base base class instead of m_saved_var directly.
	(scoped_restore_tmpl::scoped_restore_tmpl(T*, T2)): Likewise.
	(scoped_restore_tmpl::scoped_restore_tmpl(const
	scoped_restore_tmpl<T>&)): Likewise.
	(scoped_restore_tmpl::~scoped_restore_tmpl): Use the saved_var
	method.
	(scoped_restore_tmpl::saved_var): New method.
	(scoped_restore_tmpl::m_saved_var): Delete.
	* inferior.h (inferior::detaching): Now a bool.
	* infrun.c (prepare_for_detach): Use a scoped_restore instead of a
	cleanup.
	* unittests/scoped_restore-selftests.c: New file.

gdb/unittests/scoped_restore-selftests.c

@@ -0,0 +1,110 @@
+/* Self tests for scoped_restore for GDB, the GNU debugger.
+
+   Copyright (C) 2017 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/>.  */
+
+#include "defs.h"
+#include "selftest.h"
+#include "common/scoped_restore.h"
+
+namespace selftests {
+namespace scoped_restore_tests {
+
+struct Base {};
+struct Derived : Base {};
+
+static int global;
+
+/* Check that we can return a scoped_restore from a function.  Below
+   we'll make sure this does the right thing.  */
+static scoped_restore_tmpl<int>
+make_scoped_restore_global (int value)
+{
+  return make_scoped_restore (&global, value);
+}
+
+static void
+run_tests ()
+{
+  /* Test that single-argument make_scoped_restore restores the
+     original value on scope exit.  */
+  {
+    int integer = 0;
+    {
+      scoped_restore restore = make_scoped_restore (&integer);
+      SELF_CHECK (integer == 0);
+      integer = 1;
+    }
+    SELF_CHECK (integer == 0);
+  }
+
+  /* Same, with two-argument make_scoped_restore.  */
+  {
+    int integer = 0;
+    {
+      scoped_restore restore = make_scoped_restore (&integer, 1);
+      SELF_CHECK (integer == 1);
+    }
+    SELF_CHECK (integer == 0);
+  }
+
+  /* Test releasing a scoped_restore.  */
+  {
+    int integer = 0;
+    {
+      scoped_restore restore = make_scoped_restore (&integer, 1);
+      SELF_CHECK (integer == 1);
+      restore.release ();
+    }
+    /* The overridden value should persist.  */
+    SELF_CHECK (integer == 1);
+  }
+
+  /* Test creating a scoped_restore with a value of a type convertible
+     to T.  */
+  {
+    Base *base = nullptr;
+    Derived derived;
+    {
+      scoped_restore restore = make_scoped_restore (&base, &derived);
+
+      SELF_CHECK (base == &derived);
+    }
+    SELF_CHECK (base == nullptr);
+  }
+
+  /* Test calling a function that returns a scoped restore.  Makes
+     sure that if the compiler emits a call to the copy ctor, that we
+     still do the right thing.  */
+  {
+    {
+      SELF_CHECK (global == 0);
+      scoped_restore restore = make_scoped_restore_global (1);
+      SELF_CHECK (global == 1);
+    }
+    SELF_CHECK (global == 0);
+  }
+}
+
+} /* namespace scoped_restore_tests */
+} /* namespace selftests */
+
+void
+_initialize_scoped_restore_selftests ()
+{
+  register_self_test (selftests::scoped_restore_tests::run_tests);
+}