<flat_map> <flat_set>: Add _NODISCARD, remove nested _Clear_guard, add precondition checks etc. (#5953)

* Add _NODISCARD to flat_map and flat_set.

* Don't query the nothrow_move_constructibility of flat_map::key_compare when it is only copied.

* flat_map::_Extract_using_allocator should be rvalue-ref qualified.

* Make flat_map::extract consistent with flat_set::extract.

* flat_set::_Erase should not pessimize performance for different _Ty and _Kty.

* Some libcxx tests have to be FAILed due to throwing move constructor of comparator.

* Remove nested _Clear_guards form flat_map

* Make newlines between functions in <flat_set> consistent (remove between
short repetitive functions)

* Add precondition checks to flat_map

* consistently use typedef key_compare instead of template parameter _Compare in flat_map code.

* Fix formatting.

* _Flat_map_base::_Is_sorted should not be template.

* Wrap _Flat_map_base::_Key_compare with _Pass_fn when calling standard algorithms (e.g. lower_bound)

* Correct formatting.

* Introduce a helper function to call _Pass_fn in flat_map.

* Start to unify implementation of deduplication between flat_map and flat_set.

* Make begin/end available when including <flat_map>

* Reference correct standard WP in comments.

* Correct formatting.

* Use ranges::sort instead of std::sort.

* Mention that FAILed libcxx tests are due to different resolution of LWG-2227

* Fix/update citations.

* Consistently use the `_Always_clear` name.

* All shall love `adjacent_find()` and despair.

* Categorize LWG-2227 divergence as "likely bogus tests", not "missing STL features".

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Co-authored-by: Stephan T. Lavavej <stl@microsoft.com>

Author: vmichal