[llvm] [Clang] Add __builtin_vectorelements to get number of elements in vector (PR #69010)

[Erich Keane via llvm-commits]

erichkeane wrote:

> > Also, needs a release note.
> 
> How do I do this? I cannot seem to find documentation on the process...

See docs/ReleaseNotes.rst.

https://github.com/llvm/llvm-project/pull/69010