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

[Sam James via cfe-commits]

thesamesam wrote:

> > Yes, please, unless there's a strong reason not to, consider `-fhardened`.
> 
> `-fhardened` also enables a few compiler flags. I think it would be better to have a separate flag that only affects the library and hopefully in the future start supporting `-fhardened` flag that would have an aligned behavior between GCC and Clang (including enabling equivalent compiler flags where possible).

Then why can't people just use `-D` to set the single macro?

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