nightly

Merge #4065

4065: Complete unqualified enum names in patterns and expressions r=matklad a=nathanwhit

This PR implements the completion described in #4014.
The result looks like so for patterns:
<img width="542" alt="Screen Shot 2020-04-20 at 3 53 55 PM" src="https://user-images.githubusercontent.com/17734409/79794010-8f529400-831f-11ea-9673-f838aa9bc962.png">

and for `expr`s:
<img width="620" alt="Screen Shot 2020-04-21 at 3 51 24 PM" src="https://user-images.githubusercontent.com/17734409/79908784-d73ded80-83e9-11ea-991d-921f0cb27e6f.png">


I'm not confident that the completion text itself is very robust, as it will unconditionally add completions for enum variants with the form `Enum::Variant`. This means (I believe) it would still suggest `Enum::Variant` even if the local name is changed i.e. `use Enum as Foo` or the variants are brought into scope such as through `use Enum::*`.

Co-authored-by: nathanwhit <[email protected]>