[OpenCL validation] Recursion

From the environment spec:
```
Recursion is not supported. The static function call graph for an entry point must not contain
cycles.
```

Start by locating the OpenCL validation entry point and the code that analyzes static function call graphs. Confirm how recursive cycles are currently handled, then add coverage for an entry point whose call graph contains a cycle and verify that validation rejects it according to the quoted environment specification.

Written by the indexing model from the issue text.