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

[Manna]

Summary

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

[Manna]

Thank you @erichkeane for reviews!

clang/lib/AST/Type.cpp

if (const BuiltinType *BT = getAs<BuiltinType>()) {
     }
   }
   return false;
 }
 
 QualType Type::getSveEltType(const ASTContext &Ctx) const {
   assert(isVLSTBuiltinType() && "unsupported type!");
 
-  const BuiltinType *BTy = getAs<BuiltinType>();
+  const BuiltinType *BTy = castAs<BuiltinType>();
   if (BTy->getKind() == BuiltinType::SveBool)
     // Represent predicates as i8 rather than i1 to avoid any layout issues.
     // The type is bitcasted to a scalable predicate type when casting between
     // scalable and fixed-length vectors.
     return Ctx.UnsignedCharTy;
   else
     return Ctx.getBuiltinVectorTypeInfo(BTy).ElementType;
 }