Refine CPPLINT script to not report __CPROVER_ usage outside of strings

[angelhof]

This commit refines the CPPLINT script to only report __CPROVER_ usages in strings (and not in function names for example).

I also removed several unnecessary NOLINT comments.

[codecov-io]

Codecov Report

Merging #5000 into develop will decrease coverage by 0.18%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #5000      +/-   ##
===========================================
- Coverage    69.43%   69.24%   -0.19%     
===========================================
  Files         1310     1309       -1     
  Lines       108736   108453     -283     
===========================================
- Hits         75497    75096     -401     
- Misses       33239    33357     +118

Impacted Files	Coverage Δ
unit/testing-utils/require_expr.cpp	0% <0%> (-34.38%)	⬇️
...bytecode_convert_method/convert_invoke_dynamic.cpp	76.44% <0%> (-23.08%)	⬇️
...nit/java-testing-utils/require_goto_statements.cpp	92.61% <0%> (-6.05%)	⬇️
src/goto-programs/slice_global_inits.cpp	88% <0%> (-3.67%)	⬇️
src/util/mathematical_expr.h	90.47% <0%> (-2.63%)	⬇️
src/goto-symex/goto_symex.cpp	96.77% <0%> (-2.39%)	⬇️
src/util/namespace.cpp	78.33% <0%> (-1.67%)	⬇️
...c/goto-harness/memory_snapshot_harness_generator.h	87.5% <0%> (-1.39%)	⬇️
src/ansi-c/c_typecheck_expr.cpp	72.52% <0%> (-1.27%)	⬇️
src/util/std_code.h	91.18% <0%> (-1.13%)	⬇️
... and 52 more

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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 60c49fd).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/122513922
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[thk123]

I won't block this PR as cpplint.py doesn't even have any tests running in CI - but I think it'd be really great to have a few tests of that regex so it is clearer what it does and does not match