Linking: accept the same contract from multiple translation units

[tautschnig]

Contracts stored in header files may end up in multiple translation units. Linking such translation units was rejected in the initial contracts-in-symbols implementation. Such contracts, however, are equal and we can just keep a single copy of them.

Fixes: #7156

• 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

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

Coverage data is based on head (8efe88d) compared to base (8115bb9).
Patch coverage: 77.58% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7225   +/-   ##
========================================
  Coverage    77.89%   77.89%           
========================================
  Files         1616     1616           
  Lines       186791   186800    +9     
========================================
+ Hits        145498   145511   +13     
+ Misses       41293    41289    -4     

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 40 more

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