Expand std::iter_swap and remove _Swap_adl

[frederick-vs-ja]

Also

• Move std::iter_swap from <utility> to <algorithm>.
• In _Partition_by_median_guess_unchecked, only evaluate _Prev_iter(_Glast) once.

Edit: I attempted to remove _Swap_adl directly but this failed due to T::swap member functions.

Edit: I should write using _STD swap;.

Fixes #2692.

In WG21-N4140 and WG21-N4659, the std::swap overload for arrays relied on std::swap_ranges (N4140 [utility.swap]/6, N4659 [utility.swap]/6), while std::swap_ranges recursively relied on std::swap (N4140 [alg.swap]/2, N4659 [alg.swap]/2). So I believe even if old standards allowed specializing std::iter_swap with making its behavior different (which is not a fact IMO), std::swap shouldn't be affected.

std::iter_swap is only specified to be used for std::reverse (N4140 [alg.reverse]/2, N4659 [alg.reverse]/2, N4868 [alg.reverse]/2). As said in #2692 (comment), I think it should be OK to inline std::iter_swap in std::reverse even in old modes.

[StephanTLavavej]

Thanks, this is a nice improvement for debuggability! I pushed a commit to remove a couple of using-declarations.

[StephanTLavavej]

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

[CaseyCarter]

Thanks for swapping calls to _Swap_adl with direct calls to swap!