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

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.