autodoc_typehints does not effect to overloaded callables

[tk0miya]

Describe the bug
autodoc_typehints does not effect to overloaded callables.

To Reproduce

# in conf.py
autodoc_typehints = 'none'

# in index.rst
.. automodule:: example
   :members:
   :undoc-members:

# in example.py
from typing import overload


@overload
def foo(x: int) -> int:
    ...


@overload
def foo(x: float) -> float:
    ...


def foo(x):
    return x

Expected behavior
All typehints for overloaded callables are obeyed autodoc_typehints setting.

Your project
No

Screenshots
No

Environment info

• OS: Mac
• Python version: 3.8.2
• Sphinx version: 3.1.0dev
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: No

Additional context
No

[tk0miya]

I posted a PR #8282 to support the case of autodoc_typehints='none'. But additional works are still needed for the case of autodoc_typehints='description'.