fix: keep comments for 'Fill match arms'

[A4-Tacks]

Fixes #21718

Example

enum E { A, B, C }

fn foo(t: E) -> i32 {
    match $0t {
        // variant a
        E::A => 2
        // comment on end
    }
}

Before this PR

enum E { A, B, C }

fn foo(t: E) -> i32 {
    match t {
        E::A => 2,
        E::B => ${1:todo!()},
        E::C => ${2:todo!()},$0
    }
}

After this PR

enum E { A, B, C }

fn foo(t: E) -> i32 {
    match t {
        // variant a
        E::A => 2,
        // comment on end
        E::B => ${1:todo!()},
        E::C => ${2:todo!()},$0
    }
}