[llvm] [BasicAA] Treat returns_twice functions as clobbering unescaped objects (PR #117902)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic approved this pull request.

The whole "setjmp doesn't have to preserve local variables" thing is an artifact of ancient C compiler technology where the compiler didn't actually understand setjmp semantics.  Given a modern setjmp-aware compiler, it shouldn't be a thing; we have the ability to correctly preserve local variables.

That said, given the current state of returns_twice, carving out an exception for alloca seems okay.

https://github.com/llvm/llvm-project/pull/117902