Followup to #2942.

I'm implementing WG21-P2590R2 Explicit Lifetime Management for MSVC (as wrappers around the intrinsics implemented by @JonCavesMSFT in internal MSVC-PR-703299 on 2026-01-31). When the necessary builtins become available for Clang and EDG, we should finish the implementation.

• P2590R2: Explicit lifetime management llvm/llvm-project#105234
• VSO-2846756 EDG needs to implement __builtin_start_lifetime_as and __builtin_start_lifetime_as_array intrinsics