Default values of overloaded functions are wrongly rendered as string literal

[tk0miya]

Describe the bug
Default values of overloaded functions are wrongly rendered as string literal

To Reproduce

    @overload
    async def get_shared_api_tokens(self, service_name: str = ...) -> Dict[str, str]:
        ...

    @overload
    async def get_shared_api_tokens(self, service_name: None = ...) -> Dict[str, Dict[str, str]]:
        ...

    async def get_shared_api_tokens(
        self, service_name: Optional[str] = None
    ) -> Union[Dict[str, Dict[str, str]], Dict[str, str]]:
        """Some doc"""
        # actual implementation here

The ellipsis is rendered as '...' (see #8514).

Expected behavior
The default values are rendered as is.

Your project
No

Screenshots
No

Environment info

• OS: Mac
• Python version: 3.9.1
• Sphinx version: HEAD of 3.x
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: No

Additional context
No