Add new script for updating submodules.
Add new script for updating submodules.

* Defaults to syncing LLVM to TensorFlow's conception of things.
* Pulls in TensorFlow's BUILD files.
* Not yet ready for real use (need to fix some cmake stuff and switch things in the workspace).

Closes #366

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/366 from stellaraccident:submodscript1 8d2af62a3d02dd0579309730da74afd38dad4e10
PiperOrigin-RevId: 287931571

google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,google/iree