swift234

SILGen: Minimal workaround for multiple-emission bug with Clang-importer-synthesized properties on enums.

If you extend a C/ObjC enum type to conform to ErrorType, then the synthesized _code getter ends up registered as an external decl, but appears to already get synthesized due to @slavapestov's changes. I'm not sure whether this is reliable enough that we can simply stop registering the external decls, so just to keep the build from crashing, insert a check in emitExternalDecls that we didn't already emit the decl and skip emission. Low-risk workaround for rdar://problem/24287125.