[mypyc] singledispatch made outside the main namespace

Mypyc cannot compile if there's a singledispatch function defined outside the main namespace, for example in the body of a function or class.

To Reproduce

1. Make a file with the following contents

from __future__ import annotations
from functools import singledispatch, _SingleDispatchCallable
from typing import Any


def outer() -> _SingleDispatchCallable:

    @singledispatch
    def inner(obj: object) -> Any:
        return 2

    return inner

2. Compile with mypyc

Actual Behavior

Traceback (most recent call last):
  File "mypyc/irbuild/builder.py", line 176, in accept
  File "mypy/nodes.py", line 831, in accept
  File "mypyc/irbuild/visitor.py", line 123, in visit_decorator
  File "mypyc/irbuild/function.py", line 86, in transform_decorator
  File "mypyc/irbuild/function.py", line 312, in gen_func_item
  File "mypyc/irbuild/function.py", line 351, in gen_func_ir
src/.../mwe.py:8: KeyError: <mypy.nodes.FuncDef object at 0x10a7bfb80>

The error occurs at the location of the @singledispatch.

Your Environment

• Mypy version used: '0.950'
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: Python 3.10.4
• Operating system and version: macOS Monterey 12.5.1

https://github.com/python/mypy/issues/13613#tasklist-block-c661a68b-595f-4b41-98a7-bf6bcaf6e3d9

Reproduce the failure with the provided nested singledispatch example and mypyc. Start in mypyc/irbuild/visitor.py at visit_decorator, then follow transform_decorator, gen_func_item, and gen_func_ir in mypyc/irbuild/function.py. Done means mypyc compiles the example without the reported KeyError.