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

[Sarah Spall via cfe-commits]

spall 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.

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