[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.
> 
> Yes I mean LTO internalization. We want keep them to the backend but we also want to remove them if they are not used by the backend. `llvm.used` won't tell us that we can remove them since it could be specified by the users for non-amdgpu-library functions.

I wonder if we could just define another `llvm.used` similar to `llvm.compiler.used` for this special case where the variable can be thrown away by the backend.

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