[llvm] XCOFF associated metadata (PR #159096)

[Sean Fertile via llvm-commits]

mandlebug wrote:

Thanks for the reviews. I don't think `!xcoff_ref` would be appropriate. I am adding the XCOFF implementation only first to keep it simple, but as MaskRay pointed out in a comment above this ref relocation is essentially the same as an ELF NONE relocation.  With #147427 you can add the relocation to a function with the intrinsic, but not to data. I think we should follow up this PR with one that extends the metadata implementation to ELF - we may even be able to subsume that patch with this metadata since it is supported on both functions and data. In that context I was thinking `!implicit.ref` as we are taking some implicit dependency and making it explicit through a reference. 

https://github.com/llvm/llvm-project/pull/159096