goto-symex: do not rely on undocumented find_symbols(expr) behaviour

[tautschnig]

L2 re-renaming should not be applied to symbols contained in the type
(array sizes may be symbolic). Some variants of find_symbols exclude
such symbols, when others don't. (Cleanup thereof takes place in another
commit.) Either way, goto-symex should be explicit in what it wants to
process, which is really just the expression and all its operands.

• 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 #6726 (f21e9ed) into develop (9053c5f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6726   +/-   ##
========================================
  Coverage    76.85%   76.85%           
========================================
  Files         1589     1589           
  Lines       183846   183848    +2     
========================================
+ Hits        141289   141291    +2     
  Misses       42557    42557           

Impacted Files	Coverage Δ
src/goto-symex/goto_symex_state.cpp	91.71% <100.00%> (+0.04%)	⬆️

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[kroening]

There is one further aspect when "finding" symbols: bound symbols. At the very least the comments should say whether they are included or not.