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

[Lawrence Benson via cfe-commits]

lawben wrote:

As nobody has come forward in the the last two days, I'm gonna merge this now. I'll have a look a mangling this for Microsoft in a follow-up, as this may require a few changes. I'm not yet sure what has to be changed for mangling.

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