[clang] [Clang] Fix incorrect return type for `__builtin_shufflevector` (PR #154817)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic approved this pull request.

I expect this doesn't have much effect in most cases; we have a bunch of implicit conversions between vector types, so probably in most cases the vector gets converted to the right type before anyone actually tries to use it.

LGTM

https://github.com/llvm/llvm-project/pull/154817