fix: correctly sniper-print type member comments after removal of all modifiers

[slarse]

Fix #3732

This PR fixes the bug described in #3732 by keeping track of the last printed element, and allowing the backwards search for previous whitespace to bypass modifier lists that have not been printed. To illustrate the effect of the fix, here's the initial test case file:

public class TypeMemberComments {
    // field comment
    public static int NON_FINAL_FIELD = 42;

    // method comment
    public void nonStaticMethod() {

    }

    // nested type comment
    private class NonStaticInnerClass {

    }
}

The test case removes all of the modifiers from all of the type members. Before the fix, the sniper-printed result was the following:

public class TypeMemberComments {
     int NON_FINAL_FIELD = 42;

     void nonStaticMethod() {

    }

     class NonStaticInnerClass {

    }
}

Note that the comments have disappeared and that the indentation of the type members is a bit off. After the fix, the result is like so:

public class TypeMemberComments {
    // field comment
    int NON_FINAL_FIELD = 42;

    // method comment
    void nonStaticMethod() {

    }

    // nested type comment
    class NonStaticInnerClass {

    }
}

As it should!

[monperrus]

Thanks!