swift234

Add a skeleton for API Notes YAML-to-binary conversion

Reapplies r20137 with most comments addressed.

Parses a YAML file (but not the final/full format yet).

Adds an entry to the driver for the apinotes "tool". We want the tool
to be visible to the user so it has to go to the driver.

Very limited testing as of now.

Swift SVN r20173