[clang] [llvm] [clang][HLSL][SPRI-V] Add convergence intrinsics (PR #80680)

[Nathan Gauër via cfe-commits]

Keenuts wrote:

Rebases on main (almost, HEAD is slightly broken), and added back the convergence attribute.
The backend changes are ready for this intrinsic.

https://github.com/llvm/llvm-project/pull/80680