libstdc++: Strengthen exception guarantee for mdspan methods.

libstdc++: Strengthen exception guarantee for mdspan methods.

The mdspan::is_{,always}_{unique,strided,exhaustive} methods only call
their counterparts in mdspan::mapping_type. The standard specifies that
the methods of mdspan::mapping_type are noexcept, but doesn't specify if
the methods of mdspan are noexcept.

Libc++ strengthened the exception guarantee for these mdspan methods.
This commit conditionally strengthens these methods for libstdc++.

libstdc++-v3/ChangeLog:

	* include/std/mdspan (mdspan::is_always_unique): Make
	conditionally noexcept.
	(mdspan::is_always_exhaustive): Ditto.
	(mdspan::is_always_strided): Ditto.
	(mdspan::is_unique): Ditto.
	(mdspan::is_exhaustive): Ditto.
	(mdspan::is_strided): Ditto.
	* testsuite/23_containers/mdspan/layout_like.h: Make noexcept
	configurable. Add ThrowingLayout.
	* testsuite/23_containers/mdspan/mdspan.cc: Add tests for
	noexcept.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>