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

[Konstantin Varlamov via llvm-commits]

var-const wrote:

Heads-up to https://github.com/orgs/llvm/teams/libcxx-vendors -- this patch reflects our current proposal for naming hardening modes (see the [comment on the RFC](https://discourse.llvm.org/t/rfc-hardening-in-libc/73925/11)). We plan to merge this patch early next week; ideally we'd avoid changing the names after the patch is merged to minimize disruption to those of you who use the `main` branch. If you have any objection to the names / other feedback on the patch, please let us know!

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