Rework conditional disabling of copy/move constructor/assignment:

* Replace `ConditionallyCopyable` with separate `ConditionallyConstructible`
  and `ConditionallyAssignable`, which can separately disable copying and
  moving (moving = copying by default).

* Extend `ConditionallyAssignable` so that it can separately disable copying and
  moving.

* Remove `CopyableLike`. It can be expressed using `ConditionallyConstructible`
  and `ConditionallyAssignable`.

This is more verbose but also more explicit. This encourages more careful
thinking which operations are actually depended on, e.g. copy assignment of
an object holding a `T` might be implemented using copy construction of `T`,
while move assignment might be present unconditionally.

Document some purposes of `ConditionallyCopyable` and `ConditionallyAssignable`.

Prefer deriving from them privately. Defaulted special member functions already
depend on private members, this is an extension of that.

PiperOrigin-RevId: 884551257