Handle edge case in overload subtyping

[ilevkivskyi]

Fixes #495
Fixes #12379

The solution is straightforward: we already have an existing (decent) method to union similar callables, so I simply refactor the code so that it can be used in subtypes.py to handle the relevant edge case.

cc @JukkaL @hauntsaninja

[ilevkivskyi]

Oh, it looks like using make_simplified_union() here causes an infinite recursion. I will take a look.

[ilevkivskyi]

The actual problem was a reckless freshen_function_type_vars() call (such things don't play well with protocol subtype checks, as we will create a technically new type in each recursion, because of a different TypeVarId).

[github-actions]

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

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/helpers.py:990: error: Unused "type: ignore" comment  [unused-ignore]
+ aiohttp/web_response.py:500: error: Unused "type: ignore" comment  [unused-ignore]
+ aiohttp/web_response.py:509: error: Unused "type: ignore" comment  [unused-ignore]
+ aiohttp/web_request.py:327: error: Unused "type: ignore" comment  [unused-ignore]
+ aiohttp/web_request.py:336: error: Unused "type: ignore" comment  [unused-ignore]
+ aiohttp/web_app.py:147: error: Unused "type: ignore" comment  [unused-ignore]
+ aiohttp/web_app.py:162: error: Unused "type: ignore" comment  [unused-ignore]

scikit-learn (https://github.com/scikit-learn/scikit-learn)
+ sklearn/externals/array_api_extra/testing/_testing.py:389: error: Unused "type: ignore" comment  [unused-ignore]

pywin32 (https://github.com/mhammond/pywin32)
+ win32/Lib/win32timezone.py:673: error: Unused "type: ignore" comment  [unused-ignore]
+ win32/Lib/win32timezone.py:728: error: Unused "type: ignore" comment  [unused-ignore]
+ win32/Lib/win32timezone.py:739: error: Unused "type: ignore" comment  [unused-ignore]

static-frame (https://github.com/static-frame/static-frame)
+ static_frame/core/series.py:791: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/series.py:4075: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/bus.py:680: error: Unused "type: ignore" comment  [unused-ignore]

[hauntsaninja]

Wow, old issue fix!