message: Fix clang-3.8 build
  clang-3.8 complains that constructor for '...' must explicitly
  initialize the const member. Newer clangs and gcc seem to be fine with
  this, but explicitly initializing the member does not hurt.
diff: @@ -169,7 +169,7 @@ ClangModulesDeclVendorImpl::ClangModulesDeclVendorImpl(
  : m_diagnostics_engine(std::move(diagnostics_engine)),
  m_compiler_invocation(std::move(compiler_invocation)),
  m_compiler_instance(std::move(compiler_instance)),
  - m_parser(std::move(parser)) {}
  + m_parser(std::move(parser)), m_origin_map() {}
  void ClangModulesDeclVendorImpl::ReportModuleExportsHelper(
  std::set<ClangModulesDeclVendor::ModuleID> &exports,
  