Regular testing of SPIR-V using source from OpenCL CTS

We should perform regular testing of the translation to SPIR-V of IR compiled from OpenCL C source produced by the OpenCL CTS. We should use spirv-val from the SPIR-V Tools to validate the result. We should also testing translation of the SPIR-V back to LLVM IR, and unsure that the IR is valid.

Start with the OpenCL CTS source and the existing translation entry points, then review how spirv-val can validate generated SPIR-V and how SPIR-V is translated back to LLVM IR. Done means regular CTS-based coverage validates both directions and confirms the resulting SPIR-V and LLVM IR are valid.

Written by the indexing model from the issue text.