message: Emit internal enums when swift_implementation_only
  Copy the same pattern as structs and tables
  Fixes google#7542
diff: @@ -1198,7 +1198,8 @@ class SwiftGenerator : public BaseGenerator {
  void GenEnum(const EnumDef &enum_def) {
  if (enum_def.generated) return;
  - const auto is_private_access = enum_def.attributes.Lookup("private");
  + const bool is_private_access = parser_.opts.swift_implementation_only ||
  + enum_def.attributes.Lookup("private") != nullptr;
  code_.SetValue("ENUM_TYPE",
  enum_def.is_union ? "UnionEnum" : "Enum, Verifiable");
  code_.SetValue("ACCESS_TYPE", is_private_access ? "internal" : "public");
  