C#: Update the printing of extension accessor calls.

Author: michaelnebel

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

@@ -288,7 +288,11 @@ class MethodCall extends Call, QualifiableExpr, LateBindableExpr, @method_invoca
 
   override Method getQualifiedDeclaration() { result = this.getTarget() }
 
-  override string toString() { result = "call to method " + concat(this.getTarget().getName()) }
+  override string toString() {
+    if exists(this.getTargetAccessor())
+    then result = "call to extension accessor " + concat(this.getTargetAccessor().getName())
+    else result = "call to method " + concat(this.getTarget().getName())
+  }
 
   override string getAPrimaryQlClass() { result = "MethodCall" }