Define a new macro instead of conditionally inserting `_CONSTEXPR20`.

Drive-by: Combine nested conditionals.

…_library_header_units`

Discovered this has been fixed in 19.1.0 after reducing a repro to file.

We had to workaround the fact that hidden friends aren't hidden in permissive mode by stuffing `unreachable_sentinel_t`'s `operator==` into a detail namespace. Now that the changes to C++20 relational expressions are implemented, we can use a member `operator==` instead of a `friend` to more simply achieve the same effect. See https://godbolt.org/z/x5ffjdxYf for a proof-of-concept.

* Remove nested type names that are extraneous in C++20 to help us avoid depending on the existence of said nested type names.
* Always declare the "nested 5 iterator traits" (or the subset of them a given iterator provides) in canonical order at the top of the first public section in a class body.
* When the "nested 5 iterator traits" _are_ available, use their names in the semantically appropriate places in member function declarations. For example, `reference operator*() const`.

For consistency with other algorithms that take unwrapped iterators.

`_Unchecked_begin`/`_Unchecked_end` are semi-documented, so their presence doesn't necessarily indicate a specialization is library-defined. Use our `_Is_from_primary` tech, and remove the now-unused `_Has_unchecked_begin_end`.

`_Uninitialized_copy`, `_Uninitialized_copy_n`, and `_Uninitialized_move` in `<xmemory>` are internal-only, and trust that the caller has passed arguments that denote a valid range. I am not so trusting.

`_STL_INTERNAL_CHECK((_STD _Adl_verify_range(_First, _Last), true))` feels like a bit of a hack - I could be convinced to simply wrap the `_Adl_verify_range` call in `#ifdef _ENABLE_STL_INTERNAL_CHECK`.

…te a range

They must per N4988 [range.subrange.ctor] para 1 and 3. As a rule, we never verify that `[begin(meow), end(meow))` for a range `meow` actually denotes a range, so `subrange`'s constructors are the only chance we have to verify these user-supplied values.