[clang] [HLSL] Implementation of the fmod intrinsic (PR #108849)

[Zhengxing li via cfe-commits]

lizhengxing wrote:

> We should announce `__builtin_elementwise_fmod` here: `clang/docs/ReleaseNotes.rst` See @spall PR where we just did this: https://github.com/llvm/llvm-project/pull/108121/files

Done.

https://github.com/llvm/llvm-project/pull/108849