It's not clear whether we'll actually need this feature in the long
run, but we certainly need it now because non-@usableFromInline
members can (currently) satisfy public requirements when a
@usableFromInline internal type conforms to a public protocol. In
these cases, we'll treat the witnesses as present but opaque, and
clients will perform dynamic dispatch when using them even when
a generic function gets specialized.
With this, we're able to generate a textual interface for the standard
library, compile it back to a swiftmodule, and use it to build a Hello
World program!
