C#: Introduce library support for scoped.

csharp/ql/lib/semmle/code/csharp/Variable.qll

@@ -71,6 +71,11 @@ class LocalScopeVariable extends Variable, @local_scope_variable {
    */
   predicate isRef() { none() }
 
+  /**
+   * Holds if this local variable or parameter is `scoped`.
+   */
+  predicate isScoped() { scoped_annotation(this, _) }
+
   override predicate hasQualifiedName(string qualifier, string name) { none() }
 }