This makes it more explicit what the install component of a target
library is if you don't see one (and its marked as IS_SDK_OVERLAY).
Explicit in this case makes more sense, as you don't have to rely on
knowledge of how `add_swift_target_library` is implemented to understand
what component is used to install the target.
There are situations where you want to build against a libc that is out
of tree or that is not the system libc (Or for cross build scenarios).
This is a change for passing the -sdk and include paths for things like
this.
The SDK directory is now confusing as the Windows target also has a SDK
overlay. In order to make this more uniform, move the SDK directory to
Darwin which covers the fact that this covers the XNU family of OSes.
The Windows directory contains the SDK overlay for the Windows target.
