STL: Finish replacing tag dispatch with `if constexpr`

Tag dispatch is the STL's original metaprogramming technique: overload a helper function on a tag struct (e.g. input_iterator_tag versus forward_iterator_tag, or true_type versus false_type) and then call it with a tag object to select the appropriate overload. This has several downsides, though - it is somewhat verbose, it interferes with code organization, and it results in actual function calls in debug mode. (This is slower, more work to step through when debugging, and bloats object files.) C++17 if constexpr supersedes tag dispatch in almost every situation (see note below) and we're using it in new C++17-and-later code. We can use it in C++14 mode too (compilers support it unconditionally with a warning that we've suppressed).

We should finish overhauling the STL to use if constexpr.

Note: Tag dispatch works with delegating constructors, whereas if constexpr works only in function bodies. subrange's delegating constructors are a rare example of necessary tag dispatch: https://github.com/microsoft/STL/blob/f099e9c9f91187376e85385b135ac62ef933e54f/stl/inc/xutility#L3345-L3356

Start by auditing the STL implementation for remaining tag-dispatch helpers and calls, using stl/inc/xutility as the referenced example. Preserve tag dispatch where delegating constructors require it, including subrange. Done means applicable tag-dispatch code has been replaced with if constexpr without changing standard-library behavior.