[clang] [llvm] [clang][hlsl] Add atan2 intrinsic part 1 (PR #107923)

[Tex Riddell via cfe-commits]

tex3d wrote:

@farzonl 
> since this is a new builtin it would make sense to add it to `clang/docs/ReleaseNotes.rst`. There should be some examples from Josh's PRs.

I can't find the examples you're referring to.  Do you think you could point one out?  Also, what about the other builtins that were added previously, related to the same RFC?

https://github.com/llvm/llvm-project/pull/107923