Fixes Aggressive completion makes it difficult to type function types. #2494 + test case

tests/cases/fourslash/completionListNewIdentifierFunctionDeclaration.ts

@@ -0,0 +1,12 @@
+/// <reference path='fourslash.ts' />
+
+////function F(pref: (a/*1*/
+////
+////function F(pref: a/*2*/
+////
+
+test.markers().forEach((m) => {
+    goTo.position(m.position, m.fileName);
+    verify.not.completionListIsEmpty();
+    verify.completionListAllowsNewIdentifier();
+});