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

[Lawrence Benson via cfe-commits]

lawben wrote:

@tbaederr Jupp, I'm on it. I was not aware that I had to add `REQUIRES` to all the tests for it to find the header files. I assumed they are always available. See #69582.

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