Re-add generic Function type to IInjector.invoke() sygnature. (#16042)

Looks like it is used in some projects. 'Function' should be treated
as a function with '(..args: any[]) => any' signature.

types/angular/angular-tests.ts

@@ -484,7 +484,14 @@ namespace TestInjector {
         function foobar(v: boolean): number {
             return 7;
         }
-        let result: number = $injector.invoke(foobar);
+        let result = $injector.invoke(foobar);
+        if (!(typeof result === 'number')) {
+            // This fails to compile if 'result' is not exactly a number.
+            let expectNever: never = result;
+        }
+
+        let anyFunction: Function = foobar;
+        let anyResult: string = $injector.invoke(anyFunction);
     }
 }
 

types/angular/index.d.ts

@@ -1981,6 +1981,7 @@ declare namespace angular {
             instantiate<T>(typeConstructor: {new(...args: any[]): T}, locals?: any): T;
             invoke(inlineAnnotatedFunction: any[]): any;
             invoke<T>(func: (...args: any[]) => T, context?: any, locals?: any): T;
+            invoke(func: Function, context?: any, locals?: any): any;
             strictDi: boolean;
         }