[clang] [llvm] [Coroutines] Replace coro.outside.frame metadata with an intrinsic (PR #129255)

[Hans Wennborg via cfe-commits]

zmodem wrote:

This is a follow-up to the discussion on the previous PR:

- I think that discussion ended with the conclusion that we need an explicit mechanism for marking out-of-frame allocas.
- We agree it should not be metadata
- I think an intrinsic may be better than a flag directly on the alloca instruction (like `inalloca`), because we would need to teach optimizations about such a flag. For example, that an "out-of-frame" alloca is not replaceable with a regular alloca.
- I'm not entirely sure how to deprecate a metadata. I think we need to parse it for backwards compatibility, at least at the bitcode level?

https://github.com/llvm/llvm-project/pull/129255