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

[Matt Arsenault via cfe-commits]

https://github.com/arsenm commented:

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

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