Comments removed/eaten after generic trait header before body

Similarly to #4649 and #4774, comments are stripped after trait headers if after a generic parameter:

Before:
```rust 
trait Foo<T> // my comment 
{}
```
After:
```rust
trait Foo<T> {}
```

Before:
```rust 
trait Bar<T> /* My comment */ {
    fn bar();
}
```
After:
```rust
trait Bar<T> {
    fn bar();
}
```