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

[Konstantin Varlamov via llvm-commits]

var-const wrote:

@Caslyn @Artem-B Thank you for the heads-up, will merge a fix shortly.

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