[libcxx] Include __fwd/span.h in <mdspan>
This patch includes __fwd/span.h in <mdspan> so that we get the
declaration of dynamic_extent inside <mdspan>. We also clean up quite a
few tests that were manually included <span> for dynamic_extent.
This is based on feedback from #142693.
Reviewers: philnik777, ldionne
Reviewed By: philnik777
Pull Request: https://github.com/llvm/llvm-project/pull/142925
