Missing example cl_offline_compiler script

Split out from #689 following discussion with @gwawiork and @kpet

`cl_offline_compiler` is mentioned in a number of places, and is used by `test_conformance/generate_spirv_offline.py` and in `test_harness` code, the only reference which has any information is `test_harness/cl_offline_compiler-interface.txt`.

An outline of the script interface and how to use it is given [here](https://github.com/KhronosGroup/OpenCL-CTS/issues/689#issuecomment-601727567).

@gwawiork offered to prepare an example.

Read test_harness/cl_offline_compiler-interface.txt, then inspect its references in test_conformance/generate_spirv_offline.py and the test_harness code. Use the interface outline in issue #689 to define the example; done means an example cl_offline_compiler script is available and demonstrates the documented usage.

Written by the indexing model from the issue text.