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

[Lawrence Benson via cfe-commits]

lawben wrote:

@erichkeane This is my first PR to the frontend side of LLVM. Please let me know if there is something missing for a Clang PR that I should add.

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