introduce instructiont::decl_symbol()

[kroening]

This addresses the issue of the ambiguous dual-use of code_declt in both the
C front-end and the goto-program API by replacing the use of code_declt by
directly returning the symbol_exprt. The client code is simplified.

• 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

Merging #5861 (4fd73c1) into develop (4daa686) will increase coverage by 0.00%.
The diff coverage is 92.66%.

@@           Coverage Diff            @@
##           develop    #5861   +/-   ##
========================================
  Coverage    72.89%   72.89%           
========================================
  Files         1423     1423           
  Lines       154229   154242   +13     
========================================
+ Hits        112426   112440   +14     
+ Misses       41803    41802    -1     

Impacted Files	Coverage Δ
src/analyses/global_may_alias.cpp	0.00% <0.00%> (ø)
src/analyses/goto_check.cpp	88.28% <ø> (ø)
src/analyses/uninitialized_domain.cpp	0.00% <0.00%> (ø)
.../goto-instrument/goto_instrument_parse_options.cpp	58.20% <ø> (ø)
...rc/goto-instrument/goto_instrument_parse_options.h	100.00% <ø> (ø)
src/goto-programs/goto_convert_class.h	87.30% <ø> (ø)
src/goto-diff/goto_diff_parse_options.cpp	58.67% <75.00%> (ø)
src/goto-programs/goto_program.h	89.40% <80.00%> (+0.03%)	⬆️
src/goto-analyzer/goto_analyzer_parse_options.cpp	73.25% <90.90%> (ø)
src/util/std_code.h	94.00% <90.90%> (ø)
... and 36 more

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