[clang] Add option -fstdlib-hardening= (PR #78763)

[via cfe-commits]

https://github.com/cor3ntin commented:

I think this requires more documentation + a changelog entry.
(the documentation can link to a libc++ page if such thing exist, but there needs to be an explanation of the values _somewhere_. Maybe even in the help message too.
We probably do not want users to have to read the (great) RFC.

I think it make sense to explore a `-fharden` GCC compatible flag separately that would presumably map to `fast` or `extensive`. But `-fharden` has additional language/compiler effects

Making the flag compatible with libstdc++ / MSSTL in the future, if we can find a suitable mapping would be great. (@StephanTLavavej might have opinions there)

https://github.com/llvm/llvm-project/pull/78763