c++: Implement __is_invocable built-in trait

c++: Implement __is_invocable built-in trait

This patch implements built-in trait for std::is_invocable.

gcc/cp/ChangeLog:

	* cp-trait.def: Define __is_invocable.
	* constraint.cc (diagnose_trait_expr): Handle CPTK_IS_INVOCABLE.
	* semantics.cc (trait_expr_value): Likewise.
	(finish_trait_expr): Likewise.
	* cp-tree.h (build_invoke): New function.
	* method.cc (build_invoke): New function.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/has-builtin-1.C: Test existence of __is_invocable.
	* g++.dg/ext/is_invocable1.C: New test.
	* g++.dg/ext/is_invocable2.C: New test.
	* g++.dg/ext/is_invocable3.C: New test.
	* g++.dg/ext/is_invocable4.C: New test.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>