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

[Mital Ashok via cfe-commits]

MitalAshok wrote:

This is on top of #91894

[P2641R4](https://wg21.link/P2641R4)

Currently, this doesn't strictly check "whose complete object's lifetime began within `E`". A bunch of the static_asserts I have for objects under construction should be ill-formed instead of false, but some of them should be true.

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