Add support for c++17 nested namespaces

For example:
  namespace A::B { ... }
which is the equivalent to C++98 namespaces:
  namespace A { namespace B { ... } }

Examples/test-suite/common.mk

@@ -162,6 +162,7 @@ CPP_TEST_CASES += \
 	cpp_nodefault \
 	cpp_static \
 	cpp_typedef \
+	cpp17_nested_namespaces \
 	curiously_recurring_template_pattern \
 	default_args \
 	default_arg_expressions \