Remove internal symbols: keep all symbols used in contracts

[tautschnig]

In beb1353 we had chosen to only keep code-typed symbols, but indeed we also need to retain objects (only) referred to from contracts.

Also add debug-level status information of what symbols are being removed (which is information I have previously needed more than once in debugging, and it's also now the only actual use of the messaget that we already had in place).

Fixes: #7414

• 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: 78.41% // Head: 78.41% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (c4fdaf8) compared to base (206cf2a).
Patch coverage: 74.25% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7417      +/-   ##
===========================================
- Coverage    78.41%   78.41%   -0.01%     
===========================================
  Files         1659     1660       +1     
  Lines       190382   190423      +41     
===========================================
+ Hits        149280   149311      +31     
- Misses       41102    41112      +10     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
src/libcprover-cpp/api.h	100.00% <ø> (ø)
src/solvers/flattening/pointer_logic.cpp	95.06% <ø> (ø)
src/cprover/bv_pointers_wide.cpp	58.28% <26.66%> (+0.58%)	⬆️
src/libcprover-cpp/api.cpp	71.42% <63.15%> (-9.83%)	⬇️
unit/libcprover-cpp/api.cpp	72.41% <72.41%> (ø)
regression/libcprover-cpp/call_bmc.cpp	81.25% <100.00%> (+6.25%)	⬆️
src/ansi-c/c_typecheck_expr.cpp	75.78% <100.00%> (+0.02%)	⬆️
src/linking/remove_internal_symbols.cpp	100.00% <100.00%> (ø)
src/solvers/flattening/bv_pointers.cpp	86.28% <100.00%> (+0.06%)	⬆️
... and 2 more

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