[NFC][CLANG] Fix nullptr dereference issue in Type::getRVVEltType()

[Manna]

Summary

This patch uses castAs instead of getAs which will assert if the type doesn't match in clang::​Type::​getRVVEltType(clang::​ASTContext const &),

[Manna]

Thank you @erichkeane for reviews!

clang/lib/AST/Type.cpp

#include "clang/Basic/RISCVVTypes.def"
     }
   }
   return false;
 }
 
 QualType Type::getRVVEltType(const ASTContext &Ctx) const {
   assert(isRVVVLSBuiltinType() && "unsupported type!");
 
-  const BuiltinType *BTy = getAs<BuiltinType>();
+  const BuiltinType *BTy = castAs<BuiltinType>();
   return Ctx.getBuiltinVectorTypeInfo(BTy).ElementType;
 }
 
 bool QualType::isPODType(const ASTContext &Context) const {
   // C++11 has a more relaxed definition of POD.
   if (Context.getLangOpts().CPlusPlus11)
     return isCXX11PODType(Context);