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>
0129790a8f