[mypyc] Fix crash when a method overrides a decorated method

[rheard]

Fixes mypyc/mypyc#1244.

mypyc crashed when a method of a native class overrode a decorated method and the native signatures differed, for example when the override narrows the return type, or when both methods are async def with the same annotations:

class Base:
    @traced
    def get(self) -> object:
        return None

class Sub(Base):
    def get(self) -> int:
        return 1

  File "mypyc/irbuild/vtable.py", line 78, in specialize_parent_vtable
    defining_cls.glue_methods[(entry.cls, entry.name)],
KeyError: (ClassIR(name=Base, ...), 'get')

The function below the decorator is stored in ClassIR.methods, so compute_vtable() gave it a vtable entry. specialize_parent_vtable() then treated an override with a different signature like an override of a native method and looked up a glue method. There is none: a decorated method isn't in method_decls, so handle_ext_method() doesn't generate glue for overrides of it.

A decorated method doesn't need a vtable entry at all. It isn't declared as a native method, so compiled code only calls it through the Python API. Since #22157 that also holds for a property with another decorator. compute_vtable() now skips methods without a declaration. This generalizes the check added in #22121, which skipped only the undecorated function of a decorated override.

The only other code that used those entries was the __del__ lookup in generate_class(). It now uses ClassIR.get_method(), so a decorated __del__ still gets a finalizer, and still runs through the decorator.

[JukkaL]

Thanks for the fix!

[JukkaL]

Note that #22157 was merged -- do you want to update this PR before merging?