swift234

API notes: rework CMake support so that we can separate the API notes sources.

Move all of the information about API notes into the "apinotes"
subdirectory, which specifies the API notes that it provides. The
organization is such that "apinotes" can become its own separate
repository, and the compiler build will pick up those API notes that
are available and build them along with the overlays.

Swift SVN r20406