[OpenMP] Fix crash on invalid with cancel directive (#139577)
If the next token after 'cancel' is a special token, we would trigger an
assertion. We should be consuming any token, same as elsewhere in the
function.
Note, we could check for an unknown directive and do different error
recovery, but that caused too many behavioral changes for other tests in
the form of "unexpected tokens ignored" diagnostics that didn't seem
like an improvement for the test cases.
Fixes #139360