cpp/types/is invocable

@1@ Determines whether is well formed when treated as an unevaluated operand. @2@ Determines whether is well formed when treated as an unevaluated operand. @3@ Determines whether is well formed when treated as an unevaluated operand, and is known not to throw any exceptions. @4@ Determines whether is well formed when treated as an unevaluated operand, and is known not to throw any exceptions.