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

[Manna]

Summary

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

[Manna]

Rebase the patch

[Manna]

Thank you @erichkeane for reviews!

clang/lib/Sema/SemaType.cpp

S.Diag(Attr.getLoc(), diag::err_attribute_invalid_rvv_type)
         << Attr << CurType;
     Attr.setInvalid();
     return;
   }
 
   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();
 
   // The attribute vector size must match -mrvv-vector-bits.
   if (VecSize != VScale->first * MinElts * EltSize) {
     S.Diag(Attr.getLoc(), diag::err_attribute_bad_rvv_vector_size)
         << VecSize << VScale->first * llvm::RISCV::RVVBitsPerBlock;
     Attr.setInvalid();