stdlib tests: add import statements to prevent unresolved symbols when compiling StdlibUnittest with -sil-serialize-all.

rdar://problem/18917405

validation-test/stdlib/ArrayBridging.swift

@@ -14,6 +14,14 @@ import StdlibUnittest
 import Foundation
 import SlurpFastEnumeration
 
+// Also import modules which are used by StdlibUnittest internally. This is
+// needed to link all required libraries in case we serialize StdlibUnittest.
+import SwiftPrivate
+import SwiftPrivatePthreadExtras
+#if _runtime(_ObjC)
+import ObjectiveC
+#endif
+
 struct ArrayBridge_objectAtIndex_RaceTest : RaceTestWithPerTrialDataType {
   class RaceData {
     var nsa: NSArray