[clang] [llvm] [SPIRV][HLSL] Add lowering of frac to SPIR-V (PR #97111)

[Nathan Gauër via cfe-commits]

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

LGTM on the SPIR-V side, leaving MS look at the HLSL side

https://github.com/llvm/llvm-project/pull/97111