Remove LWG comments

[cpplearner]

Remove comments that cite:

• LWG-3566 Constraint recursion for operator<=>(optional<T>, U)
• "LWG issue submitted but unnumbered as of 2021-06-15" (which is LWG-3568 basic_istream_view needs to initialize value_)
• LWG-3407 Some problems with the wording changes of P1739R4
• LWG-3535 join_view::iterator::iterator_category and ::iterator_concept lie (appears once with the number, and once as "LWG issue unnumbered as of 2021-03-16")
• LWG-3498 Inconsistent noexcept-specifiers for basic_syncbuf
• LWG-3581 The range constructor makes basic_string_view not trivially move constructible
• LWG-3293 move_iterator operator+() has incorrect constraints

Several drive-by changes:

• A "per LWG issue unfiled as of 2021-06-14" comment in join_view::_Iterator is updated with the issue number (which is LWG-3569 "join_view fails to support ranges of ranges with non-default_initializable iterators")
• A new comment is added to cite LWG-3616 "LWG-3498 seems to miss the non-member swap for basic_syncbuf"
• A "per editorial or LWG issue not yet filed as of 2021-06-10" comment in <xstring> is removed. I believe that the comment refers to the bug that was fixed in cplusplus/draft@c3c40a3.

[CaseyCarter]

FYI, I pushed a tiny formatting fix.

[StephanTLavavej]

Thanks, I really appreciate your archaeological investigations here! 😻

[StephanTLavavej]

I'm mirroring this to an MSVC-internal PR; please notify me if any further changes are pushed.

[StephanTLavavej]

I've pushed a merge with main, manually resolving a conflict with #2217's refactoring in <syncstream>.

[StephanTLavavej]

Thanks again! 😻 🚀 🎉