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.
* Replace stdlib and test/stdlib 9999 availability.
macOS 9999 -> macOS 10.15
iOS 9999 -> iOS 13
tvOS 9999 -> tvOS 13
watchOS 9999 -> watchOS 6
* Restore the pre-10.15 version of public init?(_: NSRange, in: __shared String)
We need this to allow master to work on 10.14 systems (in particular, to allow PR testing to work correctly without disabling back-deployment tests).
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.
