2020-10-05

Merge #5997

5997: Better inlay hints in 'for' loops r=popzxc a=popzxc

For #5206 (one part of the fix).

This PR refines the logic of spawning an inlay hints in `for` loops. We only must provide a hint if the following criteria are met:

- User already typed `in` keyword.
- Type of expression is known and it's not unit.

**However:** I don't know why, but I was unable to make `complete_for_hint` test work. Either without or with my changes, I was always getting this test failed because no hint was spawned for the loop variable.

This change works locally, so I would really appreciate an explanation why this test isn't working now and how to fix it.

![image](https://user-images.githubusercontent.com/12111581/93024580-41a53380-f600-11ea-9bb1-1f8ac141be95.png)

Co-authored-by: Igor Aleksanov <popzxc@yandex.ru>