Merge #1432

1432: Make fill_match_arm work with trivial arm r=matklad a=ironyman

Addresses this issue https://github.com/rust-analyzer/rust-analyzer/issues/1399

One minor issue I noticed is that complete_postfix creates an arm like this

```
                match E::X {
                    <|>_ => {},
                }
```

but fill_match_arms creates arms like this
```
                     E::X => (),
```

Co-authored-by: ironyman <ironyman@users.noreply.github.com>
Co-authored-by: Changyu Li <changyl@microsoft.com>