[OpenCL][NFC] Improve OpenCL test file naming

Change "negative" into "invalid" and put "invalid" at the beginning of
the file name, following the bulk of the invalid tests in the
SemaOpenCL directory.

Use the "invalid-" prefix only for tests that contain only invalid
constructs.

Drop the "valid" suffix for CodeGen tests, as inputs in this directory
are supposed to be valid anyway.