[libc++][mdspan] Fix uglification, categorize asserts and move tests

Fixes uglification in mdspan deduction guides, which CI
did not test for until recently. The CI modification
and mdspan testing overlapped, so mdspan landed with green
CI, and the CI modification landed too.

Make most assertions in mdspan and its helper classes
trigger during a hardened build in order to catch
out of bounds access errors.

Also moves all mdspan assertions tests from libcxx/test/std
to libcxx/test/libcxx.

Differential Revision: https://reviews.llvm.org/156181