Revert D135076 "[Clang] Make offloading flags accept '-' and '--'"

This reverts commit 11adae50.
There are multiple aspects this change is not appealing.

* They conflict with JoinedOrSeparate `-o`. The old exception `-objc-*`
  should not be used an excuse.
* We generally want new options to be more rigid and avoid multiple
  spellings.
* If users get used to `-offload-*`, a misspelled `-offload-*` option still
  gets passed as `-o ffloat-*` without being detected.