C#: Move logical operation class from Operation.qll to LogicalOperation.qll.

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

@@ -56,6 +56,11 @@ class LogicalOrExpr extends BinaryLogicalOperation, @log_or_expr {
   override string getAPrimaryQlClass() { result = "LogicalOrExpr" }
 }
 
+/**
+ * A null-coalescing operation, either `x ?? y` or `x ??= y`.
+ */
+class NullCoalescingOperation extends BinaryLogicalOperation, @null_coalescing_operation { }
+
 /**
  * A null-coalescing expression, for example `s ?? ""` on line 2 in
  *

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

@@ -3,8 +3,3 @@
  */
 
 import Expr
-
-/**
- * A null-coalescing operation, either `x ?? y` or `x ??= y`.
- */
-class NullCoalescingOperation extends BinaryLogicalOperation, @null_coalescing_operation { }