Effectively mirrors LLVM-85206.
In the current standard wording, the "_`pair-like`_ but not `subrange`"
pattern is used more often than plain _`pair-like`_. So it's probably
worthwhile to carve out the `_Pair_like_non_subrange` concept.
`_Tuple_like_non_subrange` seems less worthwhile, but it should be
helpful in the constructors of  `tuple`.
Moreover, with `_Pair_like_non_subrange` carved out, we can put the
definitions of `_Pair_like` in C++20 & 23 modes together in
`<xutility>`.
`_Pair_like_non_subrange` should be also helpful for extracting the key
object from pair-like construction argument via
`_In_place_key_extract_map` (since C++23). I think this can be done in
a future PR.
Other changes: Renaming the template parameter of `subrange`'s
conversion function to `_PairLike`, making the identifier `_Pair_like
 in MSVC STL always mean the concept.