introduce code_frontend_assignt

[kroening]

Basically all frontends are using code_assignt, which is meant to be used in
goto program instructions only. This commit separates out the two uses.

• 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 #6389 (998b163) into develop (af94148) will increase coverage by 0.00%.
The diff coverage is 86.02%.

@@           Coverage Diff            @@
##           develop    #6389   +/-   ##
========================================
  Coverage    75.97%   75.98%           
========================================
  Files         1523     1523           
  Lines       164191   164221   +30     
========================================
+ Hits        124748   124780   +32     
+ Misses       39443    39441    -2     

Impacted Files	Coverage Δ
src/ansi-c/expr2c_class.h	100.00% <ø> (ø)
src/cpp/cpp_typecheck_initializer.cpp	29.26% <0.00%> (ø)
src/goto-instrument/dot.cpp	0.00% <0.00%> (ø)
src/goto-programs/goto_program.h	90.49% <ø> (+0.21%)	⬆️
src/util/allocate_objects.h	100.00% <ø> (ø)
jbmc/src/java_bytecode/code_with_references.cpp	82.35% <33.33%> (ø)
src/goto-symex/symex_main.cpp	86.00% <60.00%> (+0.03%)	⬆️
src/util/allocate_objects.cpp	90.90% <62.50%> (ø)
jbmc/src/java_bytecode/java_entry_point.cpp	89.35% <66.66%> (+0.28%)	⬆️
...rc/ansi-c/c_typecheck_gcc_polymorphic_builtins.cpp	76.58% <75.00%> (ø)
... and 19 more

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