There are two main scenarios when printing a compatibility header that
references a @cdecl enum defined in Swift code. (1) When defined in the
same module as it's used we can print the definition normally and then
reference it. (2) When used in a different mode we need to print a
forward declaration before we can reference it.
This change adds printing the forward declaration and fix an issue where
the compiler would instead print an @include of the Swift module. The
import of the Swift module would work only in a local scenario where a
compatibility header and module would be generated under the same name.
However for a distributed frameworks we do not distribute the
compatibility header so this strategy doesn't work. Relying on a forward
declaration should be more reliable in all cases but clients may need to
import the other compatibility header explicitly.
