Bind self-types in checkmember for decorated classmethods

[sterliakov]

Fixes #19023. Fixes #18993.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
+ steam/enums.py:322: error: Incompatible return value type (got "int", expected "Self")  [return-value]
- steam/state.py:2439: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Awardable[int]]"; expected "type[Self]"  [arg-type]
+ steam/state.py:2439: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Awardable[int]]"; expected "type[type[Awardable[int]]]"  [arg-type]

[sterliakov]

Okay, the steam/enums.py change is OK: there's unsupported @classproperty wrapped with a bunch of ignore comments and with unsupported ternary up the inheritance chain, enum magic doesn't work well there.