Decouple evolution of OpenCL C

We need to provide mechanisms for evolving OpenCL C independently.

I guess it should be ok to allow sources compiled with OpenCL 2.0 in OpenCL 3.0 driver? I guess the checking of device supporting the features should be done manually? Could we also allow future standards (i.e. > 3.0) in OpenCL 3.0 drivers? This would be valuable to unlock the evolution of OpenCL C independently.

Note: This was discussed on OpenCL WG call on Jun 23

Start with the OpenCL C versioning discussion in this issue, comparing the questions about OpenCL 2.0 sources, OpenCL 3.0 drivers, device feature checks, and future standards. Determine the intended version-acceptance and feature-detection rules with the OpenCL Working Group before defining the documentation changes needed.

Written by the indexing model from the issue text.