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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > An AMDGPU library function is not internalized and can be used to fullfill calls generated by LLVM passes or instruction selection.
> 
> I am confused by the description of "internalized". Do you refer to LTO internalization? You can leverage `llvm.used` to disable LTO internalization.

My guess is that the function should be considered used and then thrown away by the backend.

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