Generate-function-bodies: generate a meaningful assertion

[tautschnig]

When using --generate-function-body with "assert-false" or
"assert-false-assume-false" make sure we produce a more meaningful
description than "assertion false." Instead, make the comment say
"undefined function should be unreachable."

Fixes: #6393

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
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[codecov]

Codecov Report

Merging #6898 (577b0ec) into develop (f7a33df) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6898      +/-   ##
===========================================
- Coverage    77.81%   77.81%   -0.01%     
===========================================
  Files         1568     1568              
  Lines       179985   179975      -10     
===========================================
- Hits        140054   140044      -10     
  Misses       39931    39931              

Impacted Files	Coverage Δ
src/goto-instrument/generate_function_bodies.cpp	93.98% <ø> (-0.19%)	⬇️

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