goto_programt::get_target: fix const-ness of operations

[tautschnig]

Return a const iterator in a const context to avoid providing a means to modify instructions in a goto program despite being in a const context.

• Each commit message has a non-empty body, explaining why the change was made.
• 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

Base: 78.28% // Head: 78.28% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (943333a) compared to base (a295651).
Patch coverage: 82.75% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7322   +/-   ##
========================================
  Coverage    78.28%   78.28%           
========================================
  Files         1642     1642           
  Lines       190004   190012    +8     
========================================
+ Hits        148744   148757   +13     
+ Misses       41260    41255    -5     

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
src/goto-programs/graphml_witness.cpp	52.94% <0.00%> (-0.15%)	⬇️
src/goto-programs/interpreter.cpp	52.38% <0.00%> (+0.17%)	⬆️
src/util/expr_initializer.cpp	85.10% <80.00%> (+1.77%)	⬆️
jbmc/src/java_bytecode/expr2java.cpp	86.25% <93.75%> (ø)
src/ansi-c/c_typecheck_initializer.cpp	76.57% <100.00%> (+0.04%)	⬆️
src/ansi-c/expr2c.cpp	67.33% <100.00%> (-0.02%)	⬇️
src/goto-instrument/contracts/contracts.cpp	95.09% <100.00%> (+0.06%)	⬆️
src/goto-instrument/dump_c.cpp	79.97% <100.00%> (-0.03%)	⬇️
src/goto-programs/goto_convert.cpp	88.09% <100.00%> (ø)
... and 7 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.