New semantic analyzer: support default decorators in attrs

[ilevkivskyi]

Currently this test case fails

[case testAttrsDefaultDecorator]
defer: Yes

import attr
@attr.s
class C(object):
    x: int = attr.ib(default=1)
    y: int = attr.ib()
    @y.default
    def inc(self):
        return self.x + 1

class Yes: ...
[builtins fixtures/list.pyi]

with

main:7: error: Non-default attributes not allowed after default attributes.

See also #6438