Define std::__invoke_r for INVOKE<R>

Define std::__invoke_r for INVOKE<R>

	* include/bits/invoke.h (__invoke_r): Define new function implementing
	the INVOKE<R> pseudo-function.
	* testsuite/20_util/function_objects/invoke/1.cc: Add more tests.
	* testsuite/20_util/function_objects/invoke/2.cc: New test.

From-SVN: r271173