Merge two different applicable argument types when synthesizing corresponding argument

[A5rocks]

Fixes #18596.
Fixes #16626.

[A5rocks]

Minimizing the prefect change, I find that this PR breaks this:

from typing import Any, Callable, ParamSpec

P = ParamSpec("P")

def into(f: Callable[P, None], *args: P.args, **kwargs: P.kwargs) -> None:
    return None

class C:
    def f(self, y: bool = False, *, x: int = 42) -> None:
        return None

ex: C | Any = C()

into(ex.f, x=-1)  # E: Argument 1 to "into" has incompatible type "Callable[[bool, DefaultNamedArg(int, 'x')], None] | Any"; expected "Callable[[int], None]"

I assume that paramspec callables are somehow messed up?

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

kornia (https://github.com/kornia/kornia)
+ kornia/augmentation/container/augment.py:300: error: Unused "type: ignore" comment  [unused-ignore]
+ kornia/augmentation/container/augment.py:427: error: Unused "type: ignore" comment  [unused-ignore]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/arrays/_mixins.py:209: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/arrays/_mixins.py:217: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/indexes/multi.py:1314: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/indexes/multi.py:2427: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/plotting/_matplotlib/core.py:1605: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/plotting/_matplotlib/core.py:1772: error: Unused "type: ignore" comment  [unused-ignore]

xarray (https://github.com/pydata/xarray)
+ xarray/core/variable.py:1605: error: Unused "type: ignore" comment  [unused-ignore]

django-stubs (https://github.com/typeddjango/django-stubs): 1.40x faster (57.8s -> 41.2s in a single noisy sample)
+ django-stubs/test/client.pyi:252: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:262: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:273: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:283: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:294: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:305: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:316: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:327: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:353: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:363: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:374: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:384: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:395: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:406: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:417: error: Unused "type: ignore" comment  [unused-ignore]
+ django-stubs/test/client.pyi:428: error: Unused "type: ignore" comment  [unused-ignore]

pandas-stubs (https://github.com/pandas-dev/pandas-stubs)
+ pandas-stubs/core/indexes/multi.pyi:131: error: Unused "type: ignore" comment  [unused-ignore]

ibis (https://github.com/ibis-project/ibis)
- ibis/expr/api.py:804: error: Overloaded function implementation does not accept all possible arguments of signature 2  [misc]

jax (https://github.com/google/jax)
+ jax/experimental/jax2tf/tests/jax2tf_limitations.py:111: error: Unused "type: ignore" comment  [unused-ignore]

trio (https://github.com/python-trio/trio)
- src/trio/_core/_run.py:2743: error: Argument 1 to "run" of "Context" has incompatible type "Callable[[Any], object] | None"; expected "Callable[[Outcome[Any] | BaseException | None], object]"  [arg-type]
+ src/trio/_core/_run.py:2743: error: Argument 1 to "run" of "Context" has incompatible type "Callable[[Any], object] | None"; expected "Callable[[Any], object]"  [arg-type]