message: Update for Swift compiler terminology change.
  No user-visible functionality change.
diff: @@ -1303,7 +1303,7 @@ static bool DeserializeCompilerFlags(swift::CompilerInvocation &invocation,
  "loaded\n";
  break;
  - case swift::serialization::Status::MissingShadowedModule:
  + case swift::serialization::Status::MissingUnderlyingModule:
  error << "the swift module file is an overlay for a clang module, which "
  "can't be found\n";
  break;
  @@ -8171,7 +8171,7 @@ 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:
  