[libc++][hardening] Rework macros for enabling the hardening mode. (#70575)

1. Instead of using individual "boolean" macros, have an "enum" macro
`_LIBCPP_HARDENING_MODE`. This avoids issues with macros being
mutually exclusive and makes overriding the hardening mode within a TU
more straightforward.

2. Rename the safe mode to debug-lite.

This brings the code in line with the RFC:
https://discourse.llvm.org/t/rfc-hardening-in-libc/73925

Fixes #65101