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

[Louis Dionne via cfe-commits]

ldionne wrote:

> > > I am on the fence whether a driver option is really needed. It is a very shallow layer of extra abstraction that a curious reader has to look through. I guess I'll not object to this, if people really want to add it.
> > 
> > 
> > Setting macros that are reserved names is really unappealing for people. We want to provide a well integrated experience for library hardening and that includes providing a friendly option at the compiler level. Other sanity checking in the driver like erroring out when libc++ isn't in use is also good,
> 
> That pre-empts making it work for libstdc++ as suggested above...

Until we do something sane on libstdc++, I would error out in the Driver. As soon as we know what to do on libstdc++, we should do that instead and stop erroring out. Does that make sense? For example, in a next step we should discuss whether it makes sense for `-fstdlib-hardening={extensive,debug}` to turn on `_GLIBCXX_ASSERTIONS` on GCC. Maybe we even want that in `fast` mode? We can do anything, but we should make sure to keep the libstdc++ folks in the loop to make sure it makes sense.

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