deprecate RegExpFlags::getFlags instead of deleting it

Author: erik-krogh

java/ql/lib/semmle/code/java/security/regexp/NfaUtilsSpecific.qll

@@ -58,6 +58,14 @@ module RegExpFlags {
     root.getLiteral().isIgnoreCase()
   }
 
+  /**
+   * Gets the flags for `root`, or the empty string if `root` has no flags.
+   */
+  deprecated string getFlags(RegExpTerm root) {
+    root.isRootTerm() and
+    result = root.getLiteral().getFlags()
+  }
+
   /**
    * Holds if `root` has the `s` flag for multi-line matching.
    */