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

[Artem Belevich via cfe-commits]

Artem-B wrote:

I was thinking of implementing libm/libc for nvptx, which would produce an IR library . We'll still need to keep the functions around if they are not used explicitly, because we may need them to fulfill libcalls later in the compilation  pipeline. Sort of a libdevice replacement which can be used for libcall materialization.

But you're right, with RDC object files used for offloading it's probably not necessary.

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