<deque>: Fix single-element insertion of deque

[frederick-vs-ja]

As said in #1023 (comment), it's non-conforming to use rotate which requires swappability.

It seems to me that we should sometime use copy construction/assignment instead or moving, as done in vector. No, the requirements for deque are more relaxed.

Fixes #1023. Note that there'll be merge conflict with #4016.

[StephanTLavavej]

I thought carefully about these cases, and I think we have sufficient test coverage, so we can merge this without a second maintainer approval. I believe that resolving the merge conflict in this batch should be simple but I'll see.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

I've pushed a merge with main to resolve the conflict with how #4016 moved code around. For iterator emplace(const_iterator _Where, _Valty&&... _Val), I pasted #4022's implementation in the new location, as no other changes had been made. For iterator insert(const_iterator _Where, const _Ty& _Val), both PRs made the exact same change, so I kept the new location.

[StephanTLavavej]

Thanks for fixing this bug in the STL's favorite and best container! 😹 🐞 ✅