Remove unexpected code completion handling from ConsumeToken()

With this change tok::code_completion is finally handled exclusively as a
special token kind like other tokens that need special treatment.

All callers have been updated to use the specific token consumption methods and
the parser has a clear idea the current token isn't special by the time
ConsumeToken() gets called, so this has been unreachable for some time.

ConsumeAnyToken() behaviour is unchanged and will continue to support
unexpected code completion as part of the special token path.

This survived an amount of fuzzing and validation, but please ping the list if
you hit a code path that previously relied on the old unexpected handler and
now asserts.