ansi-c: add "" around C++ keywords

[kroening]

For consistency with the other tokens, add quotes "" around the C++ keywords in the ansi-c scanner.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 48.83721% with 44 lines in your changes missing coverage. Please review.

Project coverage is 80.36%. Comparing base (10a14c9) to head (9fbdf4b).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/ansi-c/scanner.l	48.83%	44 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8622      +/-   ##
===========================================
- Coverage    80.38%   80.36%   -0.02%     
===========================================
  Files         1686     1686              
  Lines       206772   206799      +27     
  Branches        85       84       -1     
===========================================
- Hits        166213   166200      -13     
- Misses       40559    40599      +40     

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[tautschnig]

Second commit looks good, for the first one see comment on the other PR.