C and jsil front-ends: make to_symbol return by value

[tautschnig]

Instead of creating and dummy-initialising a symbolt upfront, return a locally allocated symbol.

This removes the last users of symbolt::clear, which had maintenance risks in that it had to list all members.

• 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: 78.41% // Head: 78.42% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (296d719) compared to base (206cf2a).
Patch coverage: 62.14% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7407      +/-   ##
===========================================
+ Coverage    78.41%   78.42%   +0.01%     
===========================================
  Files         1659     1660       +1     
  Lines       190382   190378       -4     
===========================================
+ Hits        149280   149299      +19     
+ Misses       41102    41079      -23     

Impacted Files	Coverage Δ
src/ansi-c/ansi_c_declaration.h	95.00% <ø> (ø)
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
src/goto-cc/as86_cmdline.cpp	0.00% <0.00%> (ø)
src/goto-cc/as_cmdline.cpp	0.00% <0.00%> (ø)
src/goto-cc/as_mode.cpp	0.00% <0.00%> (ø)
src/goto-cc/bcc_cmdline.cpp	0.00% <0.00%> (ø)
src/jsil/jsil_convert.cpp	0.00% <0.00%> (ø)
src/jsil/jsil_entry_point.cpp	0.00% <0.00%> (ø)
src/jsil/jsil_internal_additions.cpp	0.00% <0.00%> (ø)
src/jsil/jsil_parse_tree.cpp	0.00% <0.00%> (ø)
... and 24 more

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