protect typet::subtype() const

[kroening]

Long-term readers of this sequence of PRs will be pleased to note that this is the final one for the const variant.

The non-const variant is to follow.

• 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 #7097 (4067de6) into develop (381b34d) will increase coverage by 0.01%.
The diff coverage is 87.29%.

@@             Coverage Diff             @@
##           develop    #7097      +/-   ##
===========================================
+ Coverage    77.85%   77.87%   +0.01%     
===========================================
  Files         1574     1576       +2     
  Lines       181245   181568     +323     
===========================================
+ Hits        141109   141394     +285     
- Misses       40136    40174      +38     

Impacted Files	Coverage Δ
...-sensitivity/value_set_pointer_abstract_object.cpp	88.72% <0.00%> (ø)
..._incremental/smt2_incremental_decision_procedure.h	75.00% <ø> (ø)
src/solvers/smt2_incremental/smt_solver_process.h	100.00% <ø> (ø)
src/util/type.h	98.59% <ø> (-1.41%)	⬇️
src/goto-instrument/contracts/inlining_decorator.h	34.78% <34.78%> (ø)
src/pointer-analysis/value_set_fi.cpp	69.15% <35.71%> (-0.30%)	⬇️
...c/goto-instrument/contracts/inlining_decorator.cpp	62.29% <62.29%> (ø)
src/cpp/cpp_typecheck_resolve.cpp	70.00% <62.50%> (-0.13%)	⬇️
jbmc/unit/java-testing-utils/require_type.cpp	92.18% <75.00%> (ø)
src/cpp/cpp_typecheck_expr.cpp	71.76% <77.77%> (ø)
... and 27 more

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