graphml-witnesses should not use enum values or typedef names

[tautschnig]

Make the "clean" configuration produce integers and raw types instead as neither enum values or typedef names make sense without their declarations, which aren't present in GraphML output.

• 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/
• 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

Merging #5687 (33206c1) into develop (95097a8) will increase coverage by 0.00%.
The diff coverage is 90.00%.

@@           Coverage Diff            @@
##           develop    #5687   +/-   ##
========================================
  Coverage    69.49%   69.49%           
========================================
  Files         1243     1243           
  Lines       100674   100676    +2     
========================================
+ Hits         69962    69967    +5     
+ Misses       30712    30709    -3     

Flag	Coverage Δ
cproversmt2	43.14% <90.00%> (+0.03%)	⬆️
regression	66.38% <90.00%> (+<0.01%)	⬆️
unit	32.26% <20.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/ansi-c/expr2c.cpp	69.50% <88.88%> (+0.25%)	⬆️
src/ansi-c/expr2c.h	100.00% <100.00%> (ø)

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