libstdc++: Fix std::is_nothrow_invocable_r for uncopyable prvalues [PR91456]

libstdc++: Fix std::is_nothrow_invocable_r for uncopyable prvalues [PR91456]

This is the last missing piece of PR 91456.

This also removes the only use of the C++11 version of
std::is_nothrow_invocable, which was just renamed to
__is_nothrow_invocable_lib. We can remove that now.

libstdc++-v3/ChangeLog:

	PR libstdc++/91456
	* include/std/type_traits (__is_nothrow_invocable_lib): Remove.
	(__is_invocable_impl::__nothrow_type): New member type which
	checks if the conversion can throw.
	(__is_nt_invocable_impl): Replace class template with alias
	template to __is_nt_invocable_impl::__nothrow_type.
	* testsuite/20_util/is_nothrow_invocable/91456.cc: New test.
	* testsuite/20_util/is_nothrow_convertible/value.cc: Remove
	macro used by value_ext.cc test.
	* testsuite/20_util/is_nothrow_convertible/value_ext.cc: Remove
	test for non-standard __is_nothrow_invocable_lib trait.