Add support for std::string_view in wil::str_concat

[jonwis]

I've got code that wants to combine three strings, two of which are wstring_view. It'd be nice to use wil::str_concat to build it. Right now, wil::str_concat presumes all its inputs are PCWSTR. Instead it should probably think in "view like" things internally, converting any raw pointers or pointer-holders to a counted string type.

[dunhor]

Cribbing this from something else I was working on that would likely be useful here

    // Matches types like 'std::basic_string' and 'std::basic_string_view'. Given some 'value' of type 'T', the following
    // operations can be performed:
    //      value.length()
    //      value.data()
    template <typename T, typename CharT>
    struct is_string_view_like_t
    {
        template <typename U>
        static auto evaluate(U&& value, int)
            -> wistd::enable_if_t<
                wistd::conjunction_v<
                    wistd::is_same<typename U::value_type, wistd::remove_const_t<CharT>>, // Character type must match
                    wistd::is_convertible<decltype(value.data()), wistd::add_const_t<CharT>*>, // .data() exists and returns acceptable type
                    wistd::is_same<decltype(value.length()), size_t>, // .length() exists and returns correct type
                    wistd::is_same<decltype(value.find_first_of(wistd::declval<CharT>())), size_t> // Sanity check for 'basic_string' / 'basic_string_view' types
                    >,
                wistd::true_type>;

        template <typename U>
        static wistd::false_type evaluate(U&& value, ...);

        static constexpr const bool value = decltype(evaluate(wistd::declval<T>(), 0))::value;
    };

    template <typename T, typename CharT>
    constexpr const bool is_string_view_like = is_string_view_like_t<T, CharT>::value;