goto-analyzer --(un)reachable-functions should not rely on base names

[tautschnig]

Functions are called by their symbol names, not their base names. The
output may still safely use the base name for more user-friendly
results, but call trees should not rely on a match of base names and
symbol names.

Fixes: #6330

• 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.

[martin-cs]

Looks reasonable.

[codecov]

Codecov Report

Merging #6338 (ac069cf) into develop (5e4fc98) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6338      +/-   ##
===========================================
- Coverage    75.90%   75.90%   -0.01%     
===========================================
  Files         1515     1515              
  Lines       164002   164004       +2     
===========================================
+ Hits        124482   124483       +1     
- Misses       39520    39521       +1     

Impacted Files	Coverage Δ
src/goto-analyzer/unreachable_instructions.cpp	90.95% <100.00%> (-0.42%)	⬇️

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