No systematic coverage for OpenCL C casts

The CTS does not have systematic coverage for all valid cast combinations of the following form:

```
SRCTYPE src;
DSTTYPE dst = (DSTTYPE)(src);
```

Many are covered indirectly as they are used in tests but some (e.g. int64 -> fp16) are never exercised by the CTS.

Start by locating the existing CTS tests that exercise OpenCL C casts and review which source and destination type combinations they cover. Add systematic coverage for every valid SRCTYPE-to-DSTTYPE combination in the stated cast form, including cases such as int64 to fp16, and verify the new tests through the CTS test run.

Written by the indexing model from the issue text.