SMT2 back-end: support empty compound types

[tautschnig]

Do not construct equalities over expressions the type of which has zero bit width. Also, permit pointer arithmetic over objects of such type (which was previously fixed in 6b09359 for the propositional back-end).

Fixes: #7185

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
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• n/a My commit message includes data points confirming performance improvements (if claimed).
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[codecov]

Codecov Report

Base: 77.89% // Head: 77.89% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (c3d954e) compared to base (8115bb9).
Patch coverage: 83.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7223   +/-   ##
========================================
  Coverage    77.89%   77.89%           
========================================
  Files         1616     1616           
  Lines       186791   186808   +17     
========================================
+ Hits        145498   145514   +16     
- Misses       41293    41294    +1     

Impacted Files	Coverage Δ
.../src/java_bytecode/java_bytecode_convert_class.cpp	94.18% <ø> (ø)
...src/java_bytecode/java_bytecode_convert_method.cpp	97.69% <ø> (ø)
jbmc/src/java_bytecode/java_bytecode_language.cpp	92.81% <ø> (ø)
jbmc/src/java_bytecode/java_object_factory.cpp	94.40% <ø> (ø)
jbmc/src/java_bytecode/java_string_literals.cpp	94.54% <ø> (ø)
src/analyses/cfg_dominators.h	100.00% <ø> (ø)
...le-sensitivity/value_set_pointer_abstract_object.h	100.00% <ø> (ø)
src/ansi-c/ansi_c_convert_type.h	100.00% <ø> (ø)
src/ansi-c/ansi_c_internal_additions.cpp	90.12% <ø> (ø)
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
... and 41 more

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