[clang] [LifetimeSafety] Add ExpireOriginFact to fix false positives for pointers in loops (PR #182368)

[Zhijie Wang via cfe-commits]

aeft wrote:

> If we reuse `ExpireFact`, it would need to carry both an optional `LoanID` and an optional `OriginID`, and the transfer functions would need to branch on which one is present. That feels a bit awkward with the current CRTP dispatch where each fact kind maps to one transfer function.

@usx95 @Xazax-hun  Thanks for the feedback! I updated to only expire the outermost origin and accept the multi-level pointer case as a known limitation (with a FIXME noting it needs flow-sensitive origin aliasing).

Regarding reusing `ExpireFact`, they target fundamentally different entities (Loan vs Origin) with different transfer semantics (`ExpireFact` is no-op in dataflow). To reuse it, `ExpireFact` would need to hold either a `LoanID` or an `OriginID`, and transfer functions would branch on which is present. I think separate fact kinds are cleaner here, but happy to go with reuse if you prefer.

https://github.com/llvm/llvm-project/pull/182368