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

[Sam James via cfe-commits]

thesamesam wrote:

> So if the libc++ maintainers have a strong opinion about the flag, I think more work needs to be done to ensure a good user experience.
> Personally, I think going with the macro until the flag has more functionality it controls makes sense. However, I'm also not opposed to a command line option so long as we're mindful of the user experience around it.

Right, ship it when it's polished. We can justify it more then. I'm not trying to bikeshed here - but I'm both one of the people who has to then deploy this to a lot of users, and also then _explain to other vendors_ what they should be doing, and I don't feel like this is helping the confusion if it goes in as-is. If we can get it wired up for other stdlib impls, it starts to become worth it and a net UX _win_.

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