<xhash>: Fix compiler error when dllexporting a class derived from unordered_set

[StephanTLavavej]

Fixes #1638.

Ordinarily, member functions of a class template are instantiated "on demand" when they're called. (I always remember this as being what allows list<T> to not require operator< from T, unless and until list<T>::sort() is called.) However, if a user derives from a Standard class template, and then __declspec(dllexport)s their derived class, all of the member functions of the Standard class template must be instantiated.

This is how #423 caused a regression in VS 2019 16.6. _Hash::_Multi_equal() is called only for multi containers (unordered_multimap and unordered_multiset), so it static_asserts that _Traits::_Multi is true. However, dllexport can attempt to instantiate this for the unique containers (unordered_map and unordered_set).

There are several possible fixes, but I felt that the least invasive was to make _Multi_equal() a member function template. That alone would be sufficient (i.e. template <int = 0> would work), but constraining it to exist for multi containers is simple. I left the static_assert alone, as it provides a bit of an explanation, even though it is redundant with the constraint now.

I am also adding a regression test, as this is an uncommon but recurring headache in the containers. This covers all of the containers, container adaptors, and views in the Containers clause - which are the most likely to be affected, as they sometimes have conditionally activated code like this. In theory, this test could be extended to cover any Standard type; if we encounter problems in the future, it is easy to extend, but right now having something is better than nothing.

[StephanTLavavej]

Changed the usual_matrix to the impure_matrix because the MS-internal mirror failed with error C3389: __declspec(dllexport) cannot be used with /clr:pure or /clr:safe.