Support attr converters with new semantic analyzer

[ilevkivskyi]

This is a follow up for #6435

The attrs plugin does some tricky things to support converters, in particular although the plugin is called during semantic analysis, it look like some parts belong to type checking. In particular, we need to know analyzed types of overloaded functions while still semantically analyzing top levels, which is especially tricky with the new semantic analyzer.

See also testAttrsUsingBadConverter.

[danie1k]

I've just ran with an issue with converter. Just a sample piece of code:

from distutils.version import LooseVersion
import attr

@attr.s
class Manifest:
    version: LooseVersion = attr.ib(converter=LooseVersion, default='1.0')

And the output from mypy:

tst.py:6: error: Incompatible types in assignment (expression has type "str", variable has type "LooseVersion")

[ilevkivskyi]

@danie1k your problem is unrelated to this issue and is a duplicate of python-attrs/attrs#518

[danie1k]

@ilevkivskyi Thanks! 👍
I didn't thought about searching attrs issues in this case.