Rename __range_adaptor_closure_t

[ldionne]

We discussed with @var-const today and noticed that __range_adaptor_closure_t was an incredibly bad name for this helper.

Spitballing an alternative:

template <class _Fn>
auto __make_range_adaptor_closure(_Fn&& __f) {
    struct __tmp : _Fn, __range_adaptor_closure<__tmp> {
        _LIBCPP_HIDE_FROM_ABI constexpr explicit __tmp(_Fn&& __f) : _Fn(std::move(__f)) { }
    };
    return __tmp{__f};
}

Filing an issue so we can go back to this (probably when we implement std::range_adaptor_closure).