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

[Caslyn Tonelli via llvm-commits]

Caslyn wrote:

Hi @var-const,  we are seeing errors on our downstream builders due to https://github.com/llvm/llvm-project/pull/70575#issuecomment-1802766952. Could you revert this change if a fix cannot be submitted right away?

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