[clang] [AMDGPU] add function attrbute amdgpu-lib-fun (PR #74737)

[Artem Belevich via cfe-commits]

Artem-B wrote:

This sounds like it may be useful outside of AMDGPU back-end. 
@jhuber6 this is something that may come handy for implementing general library functions.

https://github.com/llvm/llvm-project/pull/74737