[clang] [WIP][clang]: Implement a conditional lifetimebound_if builtin. (PR #125520)

[via cfe-commits]

higher-performance wrote:

w.r.t. ABI, I think the end state here would conceptually be most likely be similar to `__attribute__((diagnose_if(...)))`. Does that affect ABI?

w.r.t. False positives due to annotating the entire pack -- that's a great point. I think it's avoided by the `noexcept` approach. Any thoughts on that?

https://github.com/llvm/llvm-project/pull/125520