Fix std::visit to support arbitrary callables

Fix std::visit to support arbitrary callables

The __visitor_result_type helper didn't use std::invoke and so didn't
compile when the visitor was a pointer-to-member rather than a function
object. Use std::invoke_result instead.

	* include/std/variant (__variant_idx_cookie): Add member type.
	(__visitor_result_type): Remove.
	(__do_visit): Use invoke_result instead of __visitor_result_type.
	* testsuite/20_util/variant/visit.cc: New test.

From-SVN: r270237