Mark an effectively final field

PiperOrigin-RevId: 721050335

Author: blickly

src/com/google/javascript/jscomp/regex/RegExpTree.java

@@ -113,7 +113,7 @@ class Parser {
       int numCapturingGroups;
 
       /** The names of capturing groups in the regex expression */
-      Set<String> capturingGroupNames = new LinkedHashSet<>();
+      final Set<String> capturingGroupNames = new LinkedHashSet<>();
 
       /** The length of pattern. */
       final int limit = pattern.length();