[clang] [SPIR-V] Add clang builtin for subgroup shuffles (PR #174655)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Apparently `__builtin_hlsl_wave_read_lane_at` exists, but we probably want the generic version. I'm not familiar with HLSL, is there a reason we define these as HLSL builtins first?

https://github.com/llvm/llvm-project/pull/174655