boost/icl/concept/interval.hpp: error: unused typedef domain_compare & domain_type

Hi! Clang 9.0 generates the following warnings:

```
In file included from .../include/boost/icl/interval_map.hpp:13:
In file included from .../include/boost/icl/interval_set.hpp:14:
In file included from .../include/boost/icl/interval_base_set.hpp:24:
In file included from .../include/boost/icl/associative_interval_container.hpp:13:
In file included from .../include/boost/icl/concept/joinable.hpp:12:
.../include/boost/icl/concept/interval.hpp:111:60: error: unused typedef 'domain_compare' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_compare domain_compare;
^
.../include/boost/icl/concept/interval.hpp:110:60: error: unused typedef 'domain_type' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_type domain_type;
^
.../include/boost/icl/concept/interval.hpp:124:60: error: unused typedef 'domain_compare' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_compare domain_compare;
^
.../include/boost/icl/concept/interval.hpp:123:60: error: unused typedef 'domain_type' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_type domain_type;
^
.../include/boost/icl/concept/interval.hpp:179:60: error: unused typedef 'domain_type' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_type domain_type;
^
.../include/boost/icl/concept/interval.hpp:180:60: error: unused typedef 'domain_compare' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_compare domain_compare;
^
.../include/boost/icl/concept/interval.hpp:197:60: error: unused typedef 'domain_compare' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_compare domain_compare;
^
.../include/boost/icl/concept/interval.hpp:196:60: error: unused typedef 'domain_type' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_type domain_type;
^
.../include/boost/icl/concept/interval.hpp:292:60: error: unused typedef 'domain_type' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_type domain_type;
^
.../include/boost/icl/concept/interval.hpp:325:60: error: unused typedef 'domain_type' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_type domain_type;
^
.../include/boost/icl/concept/interval.hpp:424:60: error: unused typedef 'domain_compare' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_compare domain_compare;
^
.../include/boost/icl/concept/interval.hpp:423:60: error: unused typedef 'domain_type' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_type domain_type;
^
.../include/boost/icl/concept/interval.hpp:436:60: error: unused typedef 'domain_compare' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_compare domain_compare;
^
.../include/boost/icl/concept/interval.hpp:435:60: error: unused typedef 'domain_type' [-Werror,-Wunused-local-typedef]
typedef typename interval_traits::domain_type domain_type;
^
14 errors generated.
```