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

[Farzon Lotfi via cfe-commits]

farzonl wrote:

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.

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