Refactor trace loop heads

[thk123]

Refactoring of the common code for producing the location only steps. Feedback welcome. The goal here is to try and force the JSON and XML traces to be structurally the same

• 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.
• [na] 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).
• [na] My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5406 into develop will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #5406      +/-   ##
===========================================
- Coverage    68.19%   68.19%   -0.01%     
===========================================
  Files         1176     1177       +1     
  Lines        97527    97531       +4     
===========================================
+ Hits         66511    66512       +1     
- Misses       31016    31019       +3     

Flag	Coverage Δ
#cproversmt2	42.73% <100.00%> (-0.01%)	⬇️
#regression	65.37% <100.00%> (-0.01%)	⬇️
#unit	32.25% <30.76%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
src/goto-programs/json_goto_trace.cpp	81.57% <100.00%> (ø)
src/goto-programs/json_goto_trace.h	90.32% <100.00%> (-0.59%)	⬇️
src/goto-programs/structured_trace_util.cpp	80.00% <100.00%> (-7.50%)	⬇️
src/goto-programs/structured_trace_util.h	100.00% <100.00%> (ø)
src/goto-programs/xml_goto_trace.cpp	98.26% <100.00%> (-0.03%)	⬇️
src/util/json.cpp	82.66% <0.00%> (-1.34%)	⬇️

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[smowton]

To check: is there already regression test coverage?

[smowton]

Typo "in which loop" -> "which loop"

[thk123]

@smowton Yup - this is a refactoring of a feature introduced in #5381

[thk123]

The codecov reduction is because I made a file smaller, but the lines I removed were covered 😢 - the diff is 100% covered