[OpenCL validation] Execution model

From the environment spec:
```
The Execution Model declared in OpEntryPoint must be Kernel.
```

Start by locating the OpenCL environment validation for OpEntryPoint and the existing execution-model checks. Compare that validation with the cited environment specification; done means an OpEntryPoint using an execution model other than Kernel is rejected, with coverage in the relevant validation tests.

Written by the indexing model from the issue text.