Add test case for completion when destructuring from a union type.

Author: DanielRosenwasser

tests/cases/fourslash/completionListInObjectBindingPattern13.ts

@@ -0,0 +1,19 @@
+/// <reference path='fourslash.ts'/>
+
+////interface I {
+////    x: number;
+////    y: string;
+////    z: boolean;
+////}
+////
+////interface J {
+////    x: string;
+////    y: string;
+////}
+////
+////let { /**/ }: I | J = { x: 10 };
+
+goTo.marker();
+verify.completionListContains("x");
+verify.completionListContains("y");
+verify.not.completionListContains("z");