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

[Mital Ashok via cfe-commits]

MitalAshok wrote:

@cor3ntin I will, yes. But this probably at least needs some libc++ reviewers

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