[clang] [Clang] Add __builtin_is_within_lifetime to implement P2641R4's std::is_within_lifetime (PR #91895)

[Louis Dionne via cfe-commits]

https://github.com/ldionne commented:

I did a very cursory review and this seems good to me, the tests seem to cover what the paper was intending to provide.

https://github.com/llvm/llvm-project/pull/91895