message: Fix MissingShadowedModule -> MissingUnderlyingModule
diff: @@ -1297,7 +1297,7 @@ static const char *getImportFailureString(swift::serialization::Status status) {
  "the debugger.";
  case swift::serialization::Status::MissingDependency:
  return "The module file depends on another module that can't be loaded.";
  - case swift::serialization::Status::MissingShadowedModule:
  + case swift::serialization::Status::MissingUnderlyingModule:
  return "The module file is an overlay for a Clang module, which can't be "
  "found.";
  case swift::serialization::Status::CircularDependency:
  