C++ front-end: fix parsing of >> as closing template args

[tautschnig]

We also need to pop one token, not just replace the existing ones.

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.56%. Comparing base (0889cfa) to head (bc0ce46).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8291      +/-   ##
===========================================
+ Coverage    77.55%   77.56%   +0.01%     
===========================================
  Files         1721     1721              
  Lines       190128   190128              
  Branches     18438    18438              
===========================================
+ Hits        147448   147471      +23     
+ Misses       42680    42657      -23     

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