Fix await for inherited promise

tests/cases/conformance/async/es2017/awaitInheritedPromise_es2017.ts

@@ -0,0 +1,7 @@
+// @target: es2017
+// @strictNullChecks: true
+interface A extends Promise<string> {}
+declare var a: A;
+async function f() {
+    await a;
+}