[clang] [llvm] [HLSL] Implement elementwise popcount (PR #108121)

[Xiang Li via cfe-commits]

python3kgae wrote:

> > > This pull request is missing the appropriate text in 'ReleaseNotes.rst'. I am looking for advice on what is appropriate to put in that file, and where.
> > 
> > 
> > Here's a PR with ReleaseNotes.rst change [57f879c#diff-ec770381d76c859f5f572db789175fe44410a72608f58ad5dbb14335ba56eb97](https://github.com/llvm/llvm-project/commit/57f879cdd4c63189c569bdc1ce4e87e7342eea46#diff-ec770381d76c859f5f572db789175fe44410a72608f58ad5dbb14335ba56eb97)
> 
> Yeah I saw this example but am unsure where to put my text because the contents of that file are not the same now. Maybe the answer is to add my own section.

I see. Maybe we add HLSL Language Changes like C++ Language Changes at https://github.com/llvm/llvm-project/blob/main/clang/docs/ReleaseNotes.rst?plain=1#L106

https://github.com/llvm/llvm-project/pull/108121