Add nullability support for Xcode projects

Summary:
@public
The new version of Clang 3.6.1 deployed on the `facebook-clang-plugins` repository
3204b9291b
is patched to support the `nullability` annotation described in https://developer.apple.com/swift/blog/?id=25

**Note**: If you build Infer from source, after you pull this change, you'll need to update your checkout of `facebook-clang-plugins` and recompile clang, and this is done through the commands listed in the `INSTALL.md` file
  ./update-fcp.sh && ../facebook-clang-plugin/clang/setup.sh && ./compile-fcp.sh # run from the root of Infer repo

Test Plan: Tested on an example app containing the `nullability` annotation described by Apple in https://developer.apple.com/swift/blog/?id=25

dependencies/clang-plugin/clang-plugin-version.config

@@ -1 +1 @@
-ab60f2578bf51a8b58afa63de7cee65d7ad891c9
+24e2b11a8802be69444a702b683c1be380570bae