[clang] [Clang][SPIRV][AMDGPU] Override `supportsLibCall` for AMDGCNSPIRV (PR #143814)

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx commented:

> Well, I'm not sure if my review is gating here, if "AMDGPU does not support any libcalls at the moment" - it is what it is, no objections from my side. 

Since I am modifying the shared CodeGenInfo bit, it made sense to ask:) Also, I wasn't entirely certain that SPIRV advertising libcall support was intentional.

https://github.com/llvm/llvm-project/pull/143814