[OpenCL validation] OpTypeInt

From the environment spec:
```
For all OpTypeInt integer type-declaration instructions:
- Signedness must be 0, indicating no signedness semantics.
```

Start by locating the OpTypeInt validation entry point and its existing validation tests; the issue does not name files. Check how integer type-declaration operands are validated against the environment spec. Done means validation rejects nonzero Signedness for every OpTypeInt and regression coverage demonstrates the rule.

Written by the indexing model from the issue text.