Comments removed/eaten after generic trait header before body

[Joeoc2001]

Similarly to #4649 and #4774, comments are stripped after trait headers if after a generic parameter:

Before:

trait Foo<T> // my comment 
{}

After:

trait Foo<T> {}

Before:

trait Bar<T> /* My comment */ {
    fn bar();
}

After:

trait Bar<T> {
    fn bar();
}