libstdc++: Add more template keywords to <mdspan> for Clang

libstdc++: Add more template keywords to <mdspan> for Clang

This fixes:

include/c++/16.0.0/mdspan:1182:33: error: use 'template' keyword to treat 'mapping' as a dependent template name
 1182 |               const typename _OLayout::mapping<_OExtents>&>
      |                                        ^
include/c++/16.0.0/mdspan:1185:31: error: use 'template' keyword to treat 'mapping' as a dependent template name
 1185 |             const typename _OLayout::mapping<_OExtents>&, mapping_type>
      |                                      ^

libstdc++-v3/ChangeLog:

	* include/std/mdspan (mdspan): Add template keyword for
	dependent name.