[llvm] [TargetParser] Introduce AMDGPUTargetParser.def. NFCI. (PR #186137)

[Shilei Tian via llvm-commits]

https://github.com/shiltian approved this pull request.

Thanks for doing this. IIRC we have done something similar in comgr. We can use this there I guess. @lamb-j 

https://github.com/llvm/llvm-project/pull/186137