Simplifier type invariant: add diagnostic information

[tautschnig]

With the previous code, an invariant failure did not aid debugging.

• 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

Base: 78.28% // Head: 78.28% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d7dd40a) compared to base (9fd95ed).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7332   +/-   ##
========================================
  Coverage    78.28%   78.28%           
========================================
  Files         1642     1642           
  Lines       189992   189995    +3     
========================================
+ Hits        148734   148737    +3     
  Misses       41258    41258           

Impacted Files	Coverage Δ
...instrument/contracts/dynamic-frames/dfcc_utils.cpp	95.76% <100.00%> (+0.03%)	⬆️
src/goto-programs/goto_convert.cpp	88.09% <100.00%> (ø)
src/goto-programs/goto_program.h	90.41% <100.00%> (+0.07%)	⬆️
src/util/simplify_expr.cpp	84.42% <100.00%> (ø)
src/goto-instrument/goto_program2code.cpp	68.81% <0.00%> (-0.03%)	⬇️
src/goto-symex/symex_goto.cpp	97.38% <0.00%> (-0.01%)	⬇️

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