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

[Nathan Gauër via cfe-commits]

Keenuts wrote:

Hi, thanks for the reviews so far!
I believe we are ready to move forward on our side. Adding @llvm-beanz for the HLSL part.
The builtin I added is mostly to get something we can generate those intrinsics for. I am fine changing the name, or the implementation around it, it's just to have a wave an easy intrinsic to use/test with (no input, simplest one)

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