isOptionalParameter says unused IIFE arguments are optional

Related to adding undefined, though not strictly the same, this change
adds '?' to unused IIFE parameters in quick info.

Author: sandersn

src/compiler/checker.ts

@@ -5145,6 +5145,12 @@ namespace ts {
                 Debug.assert(parameterIndex >= 0);
                 return parameterIndex >= signature.minArgumentCount;
             }
+            const iife = getImmediatelyInvokedFunctionExpression(node.parent);
+            if (iife) {
+                return !node.type &&
+                    !node.dotDotDotToken &&
+                    indexOf((node.parent as SignatureDeclaration).parameters, node) >= iife.arguments.length;
+            }
 
             return false;
         }
@@ -14026,7 +14032,7 @@ namespace ts {
                 return getTypeOfSymbol(symbol);
             }
         }
-
+
         function getTypeAtPosition(signature: Signature, pos: number): Type {
             return signature.hasRestParameter ?
                 pos < signature.parameters.length - 1 ? getTypeOfSymbol(signature.parameters[pos]) : getRestTypeOfSignature(signature) :

tests/baselines/reference/contextuallyTypedIife.types

@@ -253,17 +253,17 @@ let eleven = (o => o.a(11))({ a: function(n) { return n; } });
 // missing arguments
 (function(x, undefined) { return x; })(42);
 >(function(x, undefined) { return x; })(42) : number
->(function(x, undefined) { return x; }) : (x: number, undefined: any) => number
->function(x, undefined) { return x; } : (x: number, undefined: any) => number
+>(function(x, undefined) { return x; }) : (x: number, undefined?: any) => number
+>function(x, undefined) { return x; } : (x: number, undefined?: any) => number
 >x : number
 >undefined : any
 >x : number
 >42 : 42
 
 ((x, y, z) => 42)();
 >((x, y, z) => 42)() : number
->((x, y, z) => 42) : (x: any, y: any, z: any) => number
->(x, y, z) => 42 : (x: any, y: any, z: any) => number
+>((x, y, z) => 42) : (x?: any, y?: any, z?: any) => number
+>(x, y, z) => 42 : (x?: any, y?: any, z?: any) => number
 >x : any
 >y : any
 >z : any

tests/cases/fourslash/quickInfoDisplayPartsIife.ts

@@ -0,0 +1,5 @@
+/// <reference path='fourslash.ts'/>
+// @strictNullChecks: true
+////var iife = (function foo/*1*/(x, y) { return x })(12);
+
+verify.quickInfoAt('1', '(local function) foo(x: number, y?: undefined): number');