[Sema] Extend GetSignedVectorType to deal with non ExtVector types

This improves some error messages which would otherwise refer to
ext_vector_type types in contexts where there are no such types.

Factored out from D25866 at reviewer's request.

Reviewers: bruno

Differential Revision: https://reviews.llvm.org/D31667



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299641 91177308-0d34-0410-b5e6-96231b3b80d8