[clang] [Clang][Sema] Emit diagnostic for __builtin_vectorelements(<SVEType>) when SVE is not available. (PR #168097)

[Alexandros Lamprineas via cfe-commits]

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

LGTM, with minor comments

https://github.com/llvm/llvm-project/pull/168097