ansi-c: more use of conditional_keyword

[kroening]

This replaces further instances of conditional keywords in the ANSI-C scanner by conditional_keyword(...).

• 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 55.81395% with 76 lines in your changes missing coverage. Please review.

Project coverage is 80.38%. Comparing base (cbdab7f) to head (d948592).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/ansi-c/scanner.l	55.81%	76 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8625      +/-   ##
===========================================
+ Coverage    80.36%   80.38%   +0.02%     
===========================================
  Files         1686     1686              
  Lines       206799   206764      -35     
  Branches        84       75       -9     
===========================================
+ Hits        166200   166215      +15     
+ Misses       40599    40549      -50     

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[tautschnig]

As discussed offline: we really should rename PARSER.cpp98 to something like PARSER.cpp98_or_later as otherwise this is very confusing.

[kroening]

Will do as separate PR.