Fix Xcode 10 errors relating to third-party (#21458)

Summary:
Fixes #20774

The new Xcode build system uses parallel execution to run build steps that don't have an obvious dependency.  Our Xcode project was written with the assumption that the **Install Third Party** build step is run _before_ compiling the `third-party` libraries.  To address this issue, this PR adds dependency information to the project to teach Xcode that `ios-install-third-party.sh` is generating the files under `third-party`.  With this additional information, Xcode correctly waits for `ios-install-third-party.sh` to finish before advancing to the compile step.

In addition to the Xcode project changes, I had to make some changes to the script `ios-install-third-party.sh` so that
1. it would always execute the `ios-configure-glog.sh` script regardless of how it was invoked
2. it would always install the libraries even if Xcode had partially created the tree or if a previous install was interrupted
Pull Request resolved: #21458

Differential Revision: D10365495

Pulled By: hramos

fbshipit-source-id: c88042583f21d2447a16f6ae2b6abb929c659a26

Author: mmccartney