Treat opencl_unroll_hint subject errors as semantic rather than parse errors

The attribute definition claimed the attribute was inheritable (which
only applies to declaration attributes) and not a statement attribute.
Further, it treats subject appertainment errors as being parse errors
rather than semantic errors, which leads to us accepting invalid code.
For instance, we currently fail to reject:

void foo() {
  int i = 1000;
  __attribute__((nomerge, opencl_unroll_hint(8)))
  if (i) { foo(); }
}

This addresses the issues by clarifying that opencl_unroll_hint is a
statement attribute and handles its appertainment checks in the
semantic layer instead of the parsing layer. This changes the output of
the diagnostic text to be more consistent with other appertainment
errors.