Fix label ids for expressions with forward refs. (mathjax/MathJax#3562)

[dpvc]

This PR fixes a problem where the ID attribute used for a tagged equation is incorrect if the equation includes a forward reference. This turns out to be due to the HandleLabel() base method skipping the setting of the tag's label during the refUpdate pass. The check for refUpdate occurs too really, and the fix is to move that check until after the label is set.

This may cause a merge conflict when the locale branch is merged, since the altered code includes a tex error call. We will have to deal with that at when it occurs.

Resolves issue mathjax/MathJax#3562.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.47%. Comparing base (a65456c) to head (ecfb01b).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1507     +/-   ##
==========================================
  Coverage    86.47%   86.47%             
==========================================
  Files          340      340             
  Lines        86125    86125             
  Branches      3206     4848   +1642     
==========================================
  Hits         74476    74476             
  Misses       11649    11649             

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