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

[Artem Belevich via llvm-commits]

Artem-B wrote:

It appears there we still have some places where the hardening mode is set to "safe". E.g.
https://github.com/llvm/llvm-project/blob/f6d487207de9795cf1ca98d7939a5ea535615abb/llvm/cmake/modules/HandleLLVMOptions.cmake#L119


https://github.com/llvm/llvm-project/pull/70575