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.
