Fixes internal VSO-2800682.

#6092 followed by #6127 implemented a small buffer optimization storing (potentially user-defined) iterators in an aggregate _Rx_capture_range_t<_BidIt>, then constructing that aggregate with _Ty{}:

This is sensitive to explicit default ctors, through Standardese that I was previously unfamiliar with. WG21-N5032 [dcl.init.aggr]/5:

For a non-union aggregate, each element that is not an explicitly initialized element is initialized as follows:

• If the element has a default member initializer ([class.mem]), the element is initialized from that initializer.
• Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list ([dcl.init.list]).
• Otherwise, the program is ill-formed.

The phrase "copy-initialized from an empty initializer list" is what rejects explicit default ctors.

Interestingly, the Standard requires forward-or-stronger iterators to be default-constructible, but only explicitly.
[forward.iterators]/1.2 "A class or pointer type X meets the Cpp17ForwardIterator requirements if [...] X meets the Cpp17DefaultConstructible requirements", then [tab:cpp17.defaultconstructible] requires only T t;, T u{};, T(), T{}.

We can fix our code by adding DMIs to activate [dcl.init.aggr]/5.1, which is a good idea in any event (helps avoid garbage-init).

Related: notepad-plus-plus/notepad-plus-plus#17816