string_constantt::to_array_expr must maintain the source location

[tautschnig]

The C front-end uses this method to create arrays from string literals.
The resulting expression must have a source location to ensure the
resulting GOTO instruction knows which source location to use.

Fixes: #6994

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• 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.
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[codecov]

Codecov Report

Merging #6995 (917f9a0) into develop (09e1041) will increase coverage by 0.01%.
The diff coverage is 92.50%.

@@             Coverage Diff             @@
##           develop    #6995      +/-   ##
===========================================
+ Coverage    77.81%   77.83%   +0.01%     
===========================================
  Files         1570     1569       -1     
  Lines       180682   180624      -58     
===========================================
- Hits        140601   140583      -18     
+ Misses       40081    40041      -40     

Impacted Files	Coverage Δ
src/util/string_constant.h	86.66% <ø> (ø)
...ncremental/smt2_incremental_decision_procedure.cpp	97.25% <ø> (ø)
src/linking/linking.cpp	59.36% <68.96%> (+0.65%)	⬆️
src/analyses/does_remove_const.cpp	100.00% <100.00%> (ø)
src/crangler/mini_c_parser.cpp	72.58% <100.00%> (+1.55%)	⬆️
src/goto-instrument/replace_calls.cpp	89.70% <100.00%> (-0.15%)	⬇️
src/goto-programs/goto_program.cpp	81.80% <100.00%> (ø)
src/goto-programs/remove_function_pointers.cpp	92.79% <100.00%> (ø)
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	88.29% <100.00%> (+0.57%)	⬆️
...ncremental/smt2_incremental_decision_procedure.cpp	94.73% <100.00%> (ø)
... and 13 more

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