Update assertion string to new name of ArithAssignBinaryOperator

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178572 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/AST/Expr.h b/include/clang/AST/Expr.h
index 377d00e..36d70d8 100644
--- a/include/clang/AST/Expr.h
+++ b/include/clang/AST/Expr.h
@@ -2900,7 +2900,7 @@
     SubExprs[LHS] = lhs;
     SubExprs[RHS] = rhs;
     assert(!isCompoundAssignmentOp() &&
-           "Use ArithAssignBinaryOperator for compound assignments");
+           "Use CompoundAssignOperator for compound assignments");
   }
 
   /// \brief Construct an empty binary operator.