[NFC][CLANG] Fix nullptr dereference issue in HandleRISCVRVVVectorBitsTypeAttr()

This patch uses castAs instead of getAs which will assert if the type doesn't match in HandleRISCVRVVVectorBitsTypeAttr(clang::QualType &, clang::ParsedAttr &, clang::Sema &)

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D151769