<xtree>: Fixes _Tree move constructor

[Arzaghi]

Fixes #1037

[miscco]

From what I read in the bug description we should also fiy the move assignment.

[StephanTLavavej]

Thanks! I've pushed a batch of changes:

• Fix typo: "constuctible" => "constructible"
• Change std:: to _STD in product code.
  • In addition to being conventional, _STD expands to ::std:: which avoids any possible ambiguity.
  • Superseded by later changes.
• In _Move_assign overloads, restore comparator copy assignment.
  • It's okay for container assignment to require comparator assignment.
  • Destroy-and-reconstruct is exception-unsafe - if reconstructing the comparator throws an exception (e.g. because the comparator attempts to allocate memory), then the container will be left with a destroyed comparator, violating its invariants.
  • It's intentional that container move construction/assignment performs comparator copy construction/assignment, respectively. The Standard (annoyingly!) doesn't talk about this clearly, but it's necessary to preserve container invariants. The issue is that moved-from containers still need to be in a valid-but-unspecified state (i.e. preconditionless member functions can be called), so they can be clear()ed (which is preconditionless) followed by inserting elements. This requires their comparators to still be usable. However, a moved-from comparator may not be usable - in particular, moved-from std::functions may be empty (and are, in our implementation). Therefore, because comparators must always be usable by the associative containers, they can't be moved-from.
  • This was unclear from the way I wrote up <xtree>: _Tree move constructor incorrectly swaps #1037; there I said "_Move_assign also copy-assigns the comparator, then calls _Swap_val anyways." and I should have said that the former was intentional but the latter was undesirable.
• Centralize code into _Swap_val_excluding_comp.
  • This code was somewhat lengthy and repeated 3 times, making it worth extracting into a helper function.
• Centralize code even more by removing _Swap_val.
  • This swaps the comparator in a slightly different order, but that's unobservable.
• Cleanup: Add top-level const to _Right_scary (a pointer).
• Comment when we're intentionally copying comparators.

[StephanTLavavej]

Thanks for fixing this longstanding bug (present since at least VS 2015 and possibly all the way back to VS 2010) and improving the usability of the associative containers! 😺