Concurrency: do not remove shared or dirty pointers from the value set

[tautschnig]

Just like we always add (and never replace) shared pointers in
goto_symex_statet::assignment, symex_dead should not remove shared or
dirty pointers.

This fixes a first problem, with further problems remaining as
indicated by the KNOWNBUG regression test.

• Each commit message has a non-empty body, explaining why the change was made.
• 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 #6121 (264c841) into develop (74e4365) will increase coverage by 0.00%.
The diff coverage is 94.44%.

@@           Coverage Diff            @@
##           develop    #6121   +/-   ##
========================================
  Coverage    75.96%   75.97%           
========================================
  Files         1508     1508           
  Lines       163292   163301    +9     
========================================
+ Hits        124052   124060    +8     
- Misses       39240    39241    +1     

Impacted Files	Coverage Δ
src/goto-symex/symex_dead.cpp	95.65% <94.44%> (-4.35%)	⬇️

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[martin-cs]

Approve but if you have time to look at the KNOWNBUG before merging, that would be great.

[tautschnig]

Approve but if you have time to look at the KNOWNBUG before merging, that would be great.

Oh, my apologies for failing to make that clear: that's fixed in #6122.