CONTRACTS: is_cprover_symbol utility function

[remi-delmas-3000]

Used by contract instrumentation to skip instrumenting CPROVER symbols with special meaning.

Extracted from #7541, can only be tested once all features are in place.

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

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b34e991) 78.50% compared to head (df1059c) 78.51%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7630   +/-   ##
========================================
  Coverage    78.50%   78.51%           
========================================
  Files         1671     1671           
  Lines       191847   191848    +1     
========================================
+ Hits        150618   150621    +3     
+ Misses       41229    41227    -2     

see 3 files with indirect coverage changes

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