Merge #6262

6262: Do not spawn redundant hints r=SomeoneToIgnore a=popzxc

Closes #5206

This is a second part of the fix (first was #5997).

This PR adds a new method to the `CompletionContext`: `no_completion_required`. If this method returns `true`, it essentially means that user is unlikely to expect any hints from the IDE at this cursor position.

Currently, checks for the following cases were added:

- Previous item is `fn`: user creates a new function, names of existing functions won't be helpful. Exception for this case is `impl Foo for Bar` -- we must suggest trait function names.
- User entered `for _ i<|>`: it's obviously going to be `in` keyword, any hints here will be confusing.

More checks may be added there later, but currently I've only figured two cases.

![no_redundant_hints](https://user-images.githubusercontent.com/12111581/96332088-da4d2a00-106a-11eb-89a1-1159ece18f9d.png)



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

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