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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

My use-case is more to be able to write functions like `is_wavefrontsize64()` in regular C++ code. This would require some way to emit builtins for these.

I believe the use-case here is a workaround for the issues caused by library ordering? I'm guessing this is related to the problems caused by prematurely optimizing out library functions that later passes wanted to depend on. 

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