Value sets must not return an empty set for nondet symbols

[tautschnig]

Even when the expression type is not immediately a pointer, some
component of the type may be a pointer. As such, it could legitimately
appear in pointer dereferencing, and we should at least add "unknown" to
the value set.

This is a follow-up fix to 3789670: the regression test included
yields a spurious counterexample in versions between 3789670 and this
bugfix commit.

• 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 #6557 (d7b38f1) into develop (e4c5999) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6557   +/-   ##
========================================
  Coverage    75.98%   75.99%           
========================================
  Files         1578     1579    +1     
  Lines       181058   181124   +66     
========================================
+ Hits        137579   137643   +64     
- Misses       43479    43481    +2     

Impacted Files	Coverage Δ
src/goto-cc/gcc_cmdline.cpp	74.80% <ø> (ø)
src/pointer-analysis/value_set.cpp	82.74% <100.00%> (-0.11%)	⬇️
...trument/contracts/havoc_assigns_clause_targets.cpp	98.07% <0.00%> (ø)
src/goto-instrument/contracts/contracts.cpp	91.28% <0.00%> (+0.15%)	⬆️

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