PR libstdc++/82254
	* include/std/type_traits (__is_invocable): Add partial specialization
	for INVOKE<void> case and remove is_void<R> check from partial
	specialization for INVOKE<R> case.
	(__is_nt_invocable_impl): New helper for is_nothrow_invocable_r.
	(is_nothrow_invocable_r): Use __is_nt_invocable_impl.
	* testsuite/20_util/is_nothrow_invocable/value.cc: Add tests for
	conversions that can throw or fail to convert. Use static assert
	strings to explain negative results.
	* testsuite/20_util/is_nothrow_invocable/value_ext.cc: Use
	is_nothrow_constructible in is_nt_invocable_conv.

From-SVN: r252977