It's not something that /ought/ to happen, but when modules aren't
quite set up properly (often by accident, thanks to header maps), we
can get into basically this situation by having the definition end up
in a header that's textually included into two different modules, or
into a module and a bridging header. Handle that the same way we
handle other redeclarations: have it show up in both modules.
rdar://problem/45646620
We treat redeclarable Clang declarations as present in every module
where they're declared, except for struct/enum/union declarations
where we only count full definitions. This logic requires going from
the imported Swift declaration back to the Clang declaration,
something that's not really possible for "synthesized declarations"
today. The only top-level synthesized declarations we have right now
are the structs we make to wrap error code enums.
The 100% correct thing to do would be to account for people defining
error code enums consistently across multiple modules. In practice,
though, error code enums are used with Objective-C (the importer's
treatment of them is tied to NSError), where redefining existing types
is very unusual. Therefore, this fix just ignores redeclarations of
error code enums, whether they're definitions or not.
rdar://problem/45414271
