stubgen: Add support for yield statements - #10745

Description

Stubgen: Support functions that yield values:

def foo():
    x = yield 123
    return ""
def bar():
    yield

will be generated as:

from collections.abc import Generator
from typing import Any

def foo() -> Generator[Any, Any, Any]: ...
def bar() -> Generator[None, None, None]: ...

Before this change, these function were annotated with a None return type.

This PR doesn't support yield from expressions (see #10744).

Test Plan

Tests for this feature and a few corner cases were added.