[mypyc] Support attributes that override properties (#14377)
Code like this is now supported by mypyc:
```
class B:
@property
def x(self) -> int:
return 0
class C(B):
x: int = 0 # Attribute overrides a property
```
The implementation generates implicit getter/setter methods for
attributes as needed and puts them in the vtable. I had to change both
the irbuild "prepare" pass (where we generate declarations), irbuild
main pass (where we generate implicit accessor IRs), and codegen (where
implicit properties aren't visible externally to CPython).
Also fix a minor mypy bug related to overriding properties and multiple
inheritance that I encountered.
This doesn't handle glue methods yet.
