[clang] [llvm] [SPIRV] Use AMDGPU ABI for AMDGCN flavoured SPIRV (PR #169865)

[Nathan Gauër via llvm-commits]

https://github.com/Keenuts approved this pull request.

LGTM for the shared SPIR-V part. Glanced at the AMD part, code LGTM, but don't know about the ABI logic so haven't checked that.

https://github.com/llvm/llvm-project/pull/169865