[llvm] [RFC] Memory Model Relaxation Annotations (PR #78569)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

I've added some reviewers that are code owners of various parts that this change affects. Please feel free to add anyone I missed.

As for the state of this PR, it's not exactly land-able as -is, but it's close. It needs some bugfixes and additional tests, but I want to do a first round of reviews for two purposes:

- Determining if high-level design changes are needed. I'd rather do these early, before I spend hours writing tests that may need to be deleted.
- Attract more attention to the RFC as a whole in order to know if this can move forward or not, as this system is important for us to implement #78572 and #78573

Finally, note that the changes to optimizations here are minimal, they're just to make sure the metadata isn't dropped too much at this time. The changes aren't focused on enabling additional optimizations or performance gains, but I posted an example in the RFC of what such a change could be. I plan to make a completely separate PR for those once this lands, so it can get proper attention.

https://github.com/llvm/llvm-project/pull/78569