libstdc++: Prepare mapping layout tests for padded layouts.

libstdc++: Prepare mapping layout tests for padded layouts.

Using the existing tests for padded layouts requires the following
changes:

 * The padded layouts are template classes. In order to be able to use
   partially specialized templates, functions need to be converted to
   structs.

 * The layout mapping tests include a check that only applies if
   is_exhaustive is static. This commit introduces a concept to check if
   is_exhaustive is a static member function.

 * Fix a test to not use a hard-coded layout_left.

The test empty.cc contains indentation mistakes that are fixed.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/mdspan/layouts/empty.cc: Fix indent.
	* testsuite/23_containers/mdspan/layouts/mapping.cc
	(test_stride_1d): Fix test.
	(test_stride_2d): Rewrite using a struct.
	(test_stride_3d): Ditto.
	(has_static_is_exhaustive): New concept.
	(test_mapping_properties): Update test.

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