Linking: do not modify source symbol table

[tautschnig]

This makes the interface more intuitive and ensures that we do not leave
behind an inconsistent symbol table.

• 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 #6548 (544fdf9) into develop (c7da320) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6548   +/-   ##
========================================
  Coverage    75.98%   75.98%           
========================================
  Files         1578     1578           
  Lines       181040   181044    +4     
========================================
+ Hits        137559   137563    +4     
  Misses       43481    43481           

Impacted Files	Coverage Δ
src/linking/linking_class.h	100.00% <ø> (ø)
src/linking/linking.cpp	59.40% <100.00%> (+0.24%)	⬆️

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