Fix mglobal.cpp build in clang-9

clang-9 reports:

```
similar/main/mglobal.cpp:210:44: error: 'report_error_uses_exception' is a protected member of 'valptridx_specialized_type_parameters<unsigned char, 90, valptridx_untyped_utilities::report_error_style::exception, valptridx_untyped_utilities::report_error_style::exception>'
template <typename T, bool = valptridx<T>::report_error_uses_exception::value>
                                           ^
similar/main/mglobal.cpp:229:16: note: in instantiation of default argument for 'instantiation_guard<dcx::active_door>' required here
template class instantiation_guard<dcx::active_door>::type::index_range_exception;
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
common/include/fwd-valptridx.h:42:2: note: constrained by protected inheritance here
    protected valptridx_specialized_types<managed_type>::type
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
common/include/cpp-valptridx.h:175:8: note: member is declared here
    using report_error_uses_exception = std::integral_constant<bool,
```

Add a `using` statement to switch `report_error_uses_exception` to
`public` access.  Nothing else uses it, and there is no design purpose
served by keeping it `protected`.

common/include/fwd-valptridx.h

@@ -61,6 +61,7 @@ class valptridx :
 	using array_base_storage_type = std::array<managed_type, array_size>;
 public:
 	class array_managed_type;
+	using typename specialized_types::report_error_uses_exception;
 
 protected:
 	using const_pointer_type = const managed_type *;