Symex slicing: fixes and cleanup

[tautschnig]

Symex slicing previously failed to track symbols appearing in types.
This commit resolves this "TODO" and replaces the implementation by
find_symbols, which knows how to get this right.

Also make sure we don't unnecessarily track goto guards when the path
condition is made part of the assertion anyway. Employ simple_slice
before doing symbol-based slicing to avoid dragging in unnecessary
dependencies from after the last assertion.

Requires #6727.

• 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).
• 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 #6728 (c7be7e5) into develop (5d74a03) will decrease coverage by 0.00%.
The diff coverage is 74.73%.

@@             Coverage Diff             @@
##           develop    #6728      +/-   ##
===========================================
- Coverage    77.78%   77.78%   -0.01%     
===========================================
  Files         1567     1567              
  Lines       179701   179770      +69     
===========================================
+ Hits        139789   139839      +50     
- Misses       39912    39931      +19     

Impacted Files	Coverage Δ
src/analyses/interval_analysis.cpp	0.00% <0.00%> (ø)
src/ansi-c/expr2c.cpp	67.63% <ø> (+0.03%)	⬆️
src/goto-instrument/accelerate/accelerate.cpp	32.28% <0.00%> (+0.33%)	⬆️
.../goto-instrument/accelerate/acceleration_utils.cpp	2.23% <0.00%> (+<0.01%)	⬆️
src/goto-instrument/accelerate/trace_automaton.cpp	0.00% <ø> (ø)
src/goto-instrument/wmm/data_dp.cpp	75.71% <ø> (ø)
src/goto-programs/goto_program.h	90.70% <ø> (ø)
src/goto-symex/symex_slice_class.h	100.00% <ø> (ø)
src/jsil/jsil_entry_point.cpp	0.00% <0.00%> (ø)
src/solvers/flattening/arrays.cpp	81.45% <ø> (ø)
... and 46 more

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