[clang] [Clang] Add support for scalable vectors in __builtin_reduce_* functions (PR #87750)

[Lawrence Benson via cfe-commits]

lawben wrote:

@efriedma-quic While there is no target independent way to create a type, it is possible to pass this type around. This is the use case I'm thinking of. So if a user has one `#ifdef` block to get the right `using VecT = ...` for the platform, they can use that `VecT` all over the place, regardless of the exact type. I'll add this to the docs and release note.

https://github.com/llvm/llvm-project/pull/87750