swift

fe74cb40 - Serialization: Deserialize opaque type xrefs from the right extension module.

Serialization: Deserialize opaque type xrefs from the right extension module. When deserializing an opaque type xref inside an extension context, we were looking incorrectly in the base module of the type being extended, rather than in the module of the extension, where the opaque type would really be. Fixes rdar://problem/51775500. This includes a small refactoring of OpaqueTypeDecl deserialization to break the inevitable cycle between deserializing the namingDecl, and the namingDecl turning around and re- deserializing its opaque return type. This is NFC but avoids some unnecessary work.