runtime: make _SwiftNativeNSError use the Hashable conformance, if available

If the Swift error wrapped in a _SwiftNativeNSError box conforms to
Hashable, the box now uses the Swift's conformance to Hashable.

Part of rdar://problem/27574348.

stdlib/public/runtime/ErrorObjectNative.cpp

@@ -17,13 +17,15 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "swift/Runtime/Config.h"
+
+#if !SWIFT_OBJC_INTEROP
+
 #include <stdio.h>
 #include "swift/Runtime/Debug.h"
 #include "ErrorObject.h"
 #include "Private.h"
 
-#if !SWIFT_OBJC_INTEROP
-
 using namespace swift;
 
 /// Determine the size and alignment of an Error box containing the given