[clang] [Clang][HIP] Add LLVM vendored device headers for `*-llvm` triple (PR #203980)

[Johannes Doerfert via cfe-commits]

jdoerfert wrote:

This is what some of the generic llvm offload headers look like:
https://github.com/jdoerfert/llvm-project/commit/d709fc5ae97c5d607f64cc25b61bfedc8571c988#diff-fcb7c2309b4900e3ba8cb72ad899d122ef4ba2ee8b9ef056a66e9e554d32de41

https://github.com/llvm/llvm-project/pull/203980