[llvm] XCOFF associated metadata (PR #159096)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Using a different metadata is the right direction. While the implementation looks fine, I am unsure whether a simple word `ref` is the best way to name this not-commonly-used feature. You should get sign-off from someone more authorative on IR like @nikic 

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