C#: Add a NullCoalescingOperation class.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/exprs/Expr.qll

@@ -252,6 +252,17 @@ class BinaryOperation extends Operation, @bin_op {
   override string toString() { result = "... " + this.getOperator() + " ..." }
 }
 
+/** A binary operation that involves a null-coalescing operation. */
+abstract private class NullCoalescingOperationImpl extends BinaryOperation { }
+
+final class NullCoalescingOperation = NullCoalescingOperationImpl;
+
+private class AddNullCoalescingExpr extends NullCoalescingOperationImpl instanceof NullCoalescingExpr
+{ }
+
+private class AddAssignCoalesceExpr extends NullCoalescingOperationImpl instanceof AssignCoalesceExpr
+{ }
+
 /**
  * A ternary operation, that is, a ternary conditional operation
  * (`ConditionalExpr`).