[ORC][C-bindings] Add access to LLJIT IRTransformLayer, ThreadSafeModule utils.

This patch was derived from Valentin Churavy's work in
https://reviews.llvm.org/D104480. It adds support for setting the transform on
an IRTransformLayer, and for accessing the IRTransformLayer in LLJIT. It also
adds access to the ThreadSafeModule::withModuleDo method for thread-safe
access to modules.

A new example has been added to show how to use these APIs to optimize a module
during materialization.

Thanks Valentin!

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D103855