Build importer overlays in advance for this test.

Fixes rdar://problem/24954005

Author: DougGregor

test/ClangModules/objc_factory_method.swift

@@ -1,4 +1,7 @@
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -target x86_64-apple-macosx10.51 -parse %s -verify
+// RUN: rm -rf %t && mkdir %t
+// RUN: %build-clang-importer-objc-overlays
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -target x86_64-apple-macosx10.51 -parse %s -verify
 
 // REQUIRES: OS=macosx
 // REQUIRES: objc_interop