Closes #12552

Stop inserting semicolon when extracting match arm

# Overview

Extracting a match arm value that has type unit into a function, when a
comma already follows the match arm value, results in an invalid (syntax
error) semicolon added between the newly generated function's generated
call and the comma.

# Example

Running this extraction

```rust
fn main() {
    match () {
        _ => $0()$0,
    };
}
```

would lead to

```rust
fn main() {
    match () {
        _ => fun_name();,
    };
}

fn fun_name() {
}
```

# Issue / Fix details

This happens because when there is no comma, rust-analyzer would simply
add the comma and wouldn't even try to evaluate whether it needs to add
a semicolon. But when the comma is there, it proceeds to evaluate
whether it needs to add a semicolon and it looks like the evaluation
logic erroneously ignores the possibility that we're in a match arm.
IIUC it never makes sense to add a semicolon when we're extracting from
a match arm value, so I've adjusted the logic to always decide against
adding a semicolon when we're in a match arm

Follow-up of rust-lang/rust-clippy#15208.

Removed some unneeded wrappings and shortened some CSS class name.

The diff is big because of one indent level getting removed. :-/

Before this PR: 1751301
With this PR: 1663634
Reduction: -5%

r? @samueltardieu

changelog: Reduce page size and number of DOM elements on clippy lints
page