swift234

APINotes: Teach the YAML converter about OS availability.

The converter silently drops the API which are available only on a OS different from the specified "target".

I've restructured the code so that we would not have to pass the parameters around.

TODO:
 - Make the traget argument required.
 - Use it in the build system when building swift
 - Add a test case for when the whole framework is missing. (Tested manually.)

Swift SVN r20464