We already supported this feature as std::__invoke<R>, for internal use.
This just adds a public version of it to <functional>.

Internal uses should continue to include <bits/invoke.h> and use
std::__invoke<R> so that they don't need to include all of <functional>.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/functional (invoke_r): Define.
	* include/std/version (__cpp_lib_invoke_r): Define.
	* testsuite/20_util/function_objects/invoke/version.cc: Check
	for __cpp_lib_invoke_r as well as __cpp_lib_invoke.
	* testsuite/20_util/function_objects/invoke/4.cc: New test.

Tested x86_64-linux. Committed to trunk.