[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.

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