Reaching definitions: distinguish recursion from function-without-body

[tautschnig]

Comparing function names addressed the problem of comparing iterators
over distinct objects (b2fba97), but resulted in possible confusion
of recursion vs calling a function that doesn't have a body.

• 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 #6720 (70779ad) into develop (eb84153) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6720      +/-   ##
===========================================
+ Coverage    76.84%   76.86%   +0.01%     
===========================================
  Files         1589     1589              
  Lines       183795   183798       +3     
===========================================
+ Hits        141238   141270      +32     
+ Misses       42557    42528      -29     

Impacted Files	Coverage Δ
src/analyses/reaching_definitions.cpp	85.00% <100.00%> (+7.33%)	⬆️
src/goto-instrument/full_slicer.cpp	94.51% <0.00%> (-1.22%)	⬇️
src/analyses/dependence_graph.cpp	90.36% <0.00%> (+0.17%)	⬆️
.../goto-instrument/goto_instrument_parse_options.cpp	70.12% <0.00%> (+0.72%)	⬆️
src/analyses/reaching_definitions.h	85.93% <0.00%> (+4.68%)	⬆️

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