[Serialization] Skip opaque types nested in skipped function bodies

Opaque types are gathered and visited separately. As with local types,
only serialize them if they are not within a skipped function body.

Fixes a crash caused when compiling the primary file, where delayed
parsing is explictly disabled.

Resolves rdar://73167790

lib/Serialization/Serialization.cpp

@@ -5273,6 +5273,11 @@ void Serializer::writeAST(ModuleOrSourceFile DC) {
     }
     
     for (auto OTD : opaqueReturnTypeDecls) {
+      // FIXME: We should delay parsing function bodies so these type decls
+      //        don't even get added to the file.
+      if (OTD->getDeclContext()->getInnermostSkippedFunctionContext())
+        continue;
+
       hasOpaqueReturnTypes = true;
       Mangle::ASTMangler Mangler;
       auto MangledName = Mangler.mangleOpaqueTypeDecl(OTD);