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

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I'm good with it if the pre-commit is, and no one comes out and really needs the mangling done in this patch.

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