[clang] [clang][SPIR-V][AMDGPU] Fix variadic aggregate ABI classification for AMDGCN (PR #216326)

[Arseniy Obolenskiy via cfe-commits]

aobolensk wrote:

> It would be good if we could move this code into the ABI library

Thanks for noting! Moved, LoC ratio became much better

https://github.com/llvm/llvm-project/pull/216326