[libc++][hardening][NFC] Rework the Lit feature for detecting the hardening mode.

Make it a multichoice string to closer mirror the CMake variable. This
allows writing `UNSUPPORTED: libcpp-hardening-mode=unchecked` rather
than `UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode`.

Differential Revision: https://reviews.llvm.org/D155906