update import-foundation-with-indexing.swift to check that we actually indexed Foundation

Author: hyp

test/Interop/Cxx/stdlib/import-foundation-with-indexing.swift

@@ -1,7 +1,11 @@
+// RUN: %empty-directory(%t)
 // RUN: %target-swift-frontend %s -c -index-system-modules -index-store-path %t -enable-experimental-cxx-interop
-//
+// RUN: ls %t/v5/units | %FileCheck %s
+
 // REQUIRES: OS=macosx
 
 import Foundation
 
-func test(d: Date) {}
+func test(d: Date) {}
+
+// CHECK: Foundation-{{.*}}.pcm