[NFC][CLANG] Fix nullptr dereference issue in HandleRISCVRVVVectorBit…

…sTypeAttr()

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

clang/lib/Sema/SemaType.cpp

@@ -8340,7 +8340,7 @@ static void HandleRISCVRVVVectorBitsTypeAttr(QualType &CurType,
   unsigned VecSize = static_cast<unsigned>(RVVVectorSizeInBits.getZExtValue());
 
   ASTContext::BuiltinVectorTypeInfo Info =
-      S.Context.getBuiltinVectorTypeInfo(CurType->getAs<BuiltinType>());
+      S.Context.getBuiltinVectorTypeInfo(CurType->castAs<BuiltinType>());
   unsigned EltSize = S.Context.getTypeSize(Info.ElementType);
   unsigned MinElts = Info.EC.getKnownMinValue();