[clang] [Clang] Implement `__builtin_start_lifetime_as` for C++23 P2590R2 (PR #196502)

[via cfe-commits]

https://github.com/Sirraide commented:

My understanding of this is that `llvm.launder.invariant.group` doesn't have the semantics that `std::start_lifetime_as` requires, and the implementation of this feature needs to wait until we have a corresponding LLVM builtin.

https://github.com/llvm/llvm-project/pull/196502