Add support for removing APIs in Darwin codegen. (#868)

* Makes the compat*NameRemapping functions just rely entirely on the
  availability rename data instead of doing non-acronym-preserving
  upcasing.
* Adds compatCommandNameRemapping to handle renamed commands.
* Adds a wasRemoved helper.
* Adds helpers to be able to do boolean logic in "if/unless".
* Adds helpers to be able to do boolean conditions on async values.