[Serialization] Handle re-export of error enums harder (#21880)
A refinement of Doug's e3207f753c from a year ago to account for
trying to reference an import-as-member error enum /through/ an
overlay when it's in a Clang module using 'export_as'. The problem is
that importing error enums synthesizes a corresponding struct, and
that struct doesn't have a Clang node and so when it isn't found in
the right module the compiler didn't know to allow it anyway.
I know, right?
(Looking at the source change might be simpler than trying to read
the above paragraph.)
I'm still not 100% sure on how this occurred in the original
(Apple-internal) projects hitting this, but I think it's just that we
handle all import-as-member types at once, and the clients were using
something else from a type that contained the error enum, and then
that resulted in the error struct's conformances getting written into
a serialized swiftmodule.
rdar://problem/47152185
