Fix and extend key extraction for unique map/set containers

[frederick-vs-ja]

Currently, the key-extracting optimization mechanism incorrectly catches some volatile-qualified glvalues (due to _Remove_cvref_t) and attempts to bind them to a const K& reference, which makes the program ill-formed. This PR make the mechanism never catch volatile glvalues, which makes volatile accepted under some conditions as specified by the standard.

In piecewise construction, when the first tuple argument is a 1-tuple, it's sometimes possible to directly treat its only tuple element as the key. Also, since C++23, it should be possible to directly get the key from suitable tuple<K, V> and array<K, 2> without additional construction. This PR attempts to extend the optimization mechanism to catch these cases.

The mechanism should also be usable for flat_set and flat_map.

Note that subrange is intentionally ignored because the corresponding get function copies (or moves) the key, which doesn't seem desired.

[StephanTLavavej]

Thanks! 😸 I pushed some minor refactorings.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for noticing and fixing this! 🦅 👁️ 🛠️

We usually changelog bugfixes as higher priority than perf improvements, but in this case volatile is extremely obscure while tuple and array aren't, so I'm going to changelog this as being primarily a perf improvement.