Interval: make public fields final

The class docs already state that Interval is immutable, but this is not enforced.
Followup to #4901

Signed-off-by: Steven Schlansker <[email protected]>

Author: stevenschlansker

runtime/Java/src/org/antlr/v4/runtime/misc/Interval.java

@@ -9,8 +9,8 @@
 public class Interval {
 	public static final Interval INVALID = new Interval(-1,-2);
 
-	public int a;
-	public int b;
+	public final int a;
+	public final int b;
 
 	public Interval(int a, int b) { this.a=a; this.b=b; }