Hide declarations steps from trace when an initialisation follows

[tautschnig]

If the front-end generated a declaration with an initialiser, then goto
conversion can mark the declaration statement (but not the
initialisation!) as hidden. This will avoid showing meaningless values
in a goto trace.

• 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 #6712 (4daadad) into develop (36d4fb8) will decrease coverage by 0.33%.
The diff coverage is 87.37%.

@@             Coverage Diff             @@
##           develop    #6712      +/-   ##
===========================================
- Coverage    77.13%   76.79%   -0.34%     
===========================================
  Files         1582     1589       +7     
  Lines       182672   183667     +995     
===========================================
+ Hits        140901   141056     +155     
- Misses       41771    42611     +840     

Impacted Files	Coverage Δ
jbmc/src/janalyzer/janalyzer_parse_options.cpp	48.74% <ø> (ø)
jbmc/src/jbmc/jbmc_parse_options.cpp	75.97% <ø> (ø)
jbmc/src/jdiff/jdiff_parse_options.cpp	67.34% <ø> (ø)
src/goto-diff/goto_diff_parse_options.h	100.00% <ø> (ø)
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
.../goto-instrument/goto_instrument_parse_options.cpp	69.39% <ø> (ø)
...rc/goto-instrument/goto_instrument_parse_options.h	100.00% <ø> (ø)
src/goto-programs/goto_check.cpp	81.39% <ø> (ø)
src/goto-programs/process_goto_program.cpp	100.00% <ø> (ø)
src/solvers/smt2/smt2_conv.cpp	67.46% <68.51%> (+0.84%)	⬆️
... and 24 more

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