Add __is_nothrow_convertible to fix std::is_nothrow_invocable_r

Add __is_nothrow_convertible to fix std::is_nothrow_invocable_r

The definition of is_nothrow_invocable in terms of is_convertible and
is_nothrow_constructible is incorrect, because a type could have an
explicit constructor that means is_nothrow_constructible is true, but
implicit conversions could use a different constructor that is
potentially-throwing.

Fix it by adding a C++11 version of C++20's is_nothrow_convertible that
only considers implicit conversions.

	* include/std/type_traits (__is_nt_convertible_helper): Define it
	unconditionally, not only for C++20.
	(__is_nothrow_convertible): Define internal trait for use in C++11.
	(__is_nt_invocable_impl: Fix by using __is_nothrow_convertible.
	(is_invocable_r_v, is_nothrow_invocable_r_v): Add missing parameter.
	* testsuite/20_util/is_nothrow_convertible/value_ext.cc: New test.
	* testsuite/20_util/is_nothrow_convertible/value.cc: Check with type
	that has nothrow explicit conversion but potentially-throwing implicit
	conversion.
	* testsuite/20_util/is_nothrow_invocable/value.cc: Likewise.
	* testsuite/20_util/is_nothrow_invocable/value_ext.cc: Fix helper
	function to only consider implicit conversions.
	* testsuite/20_util/tuple/cons/noexcept_specs.cc: Add comment.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@271171 138bc75d-0d04-0410-961f-82ee72b054a4