Warn on usage of goto-instrument --constant-propagator

[thomasspriggs]

According to the doxygen in analyses/constant_propagator.h, this is "A simple, unsound constant propagator." It is worth warning users so that it is not accidentally applied to use cases where soundness is important.

This PR relates to the following issue - #7041

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
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[codecov]

Codecov Report

Merging #7111 (d91d678) into develop (e35425e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #7111   +/-   ##
========================================
  Coverage    77.91%   77.92%           
========================================
  Files         1576     1576           
  Lines       181611   181647   +36     
========================================
+ Hits        141511   141543   +32     
- Misses       40100    40104    +4     

Impacted Files	Coverage Δ
src/util/xml.cpp	73.88% <ø> (ø)
.../goto-instrument/goto_instrument_parse_options.cpp	71.14% <100.00%> (+0.06%)	⬆️
src/cpp/cpp_convert_type.cpp	72.72% <0.00%> (-0.69%)	⬇️
src/cpp/cpp_typecheck_code.cpp	80.41% <0.00%> (-0.26%)	⬇️
src/cpp/cpp_typecheck_conversions.cpp	84.95% <0.00%> (-0.05%)	⬇️
src/cpp/template_map.cpp	57.85% <0.00%> (ø)
src/ansi-c/ansi_c_declaration.cpp	74.07% <0.00%> (ø)
src/cpp/parse.cpp	68.80% <0.00%> (+0.01%)	⬆️
src/cpp/cpp_typecheck_compound_type.cpp	86.87% <0.00%> (+0.03%)	⬆️
src/cpp/cpp_typecheck_constructor.cpp	89.18% <0.00%> (+0.08%)	⬆️
... and 5 more

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[martin-cs]

Yes but please look at #7120 which has a rather more radical take.

( Also cross referencing #6480 )

[thomasspriggs]

I have merged this PR now in order to provide a warning to users ASAP. I am happy for the warning to be removed along with the option when we are ready to merge a more comprehensive solution. Hopefully I won't have caused you too many rebasing issues.