Constructor/destructor attributes also work with forward declarations

[tautschnig]

GCC and Clang accept and apply these attributes when they are part of the forward declaration, the definition, or both.

• 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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (4af9c8a) 78.51% compared to head (b1cd9e2) 78.51%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7601   +/-   ##
========================================
  Coverage    78.51%   78.51%           
========================================
  Files         1670     1670           
  Lines       191787   191791    +4     
========================================
+ Hits        150574   150578    +4     
  Misses       41213    41213           

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.cpp	83.26% <100.00%> (+0.12%)	⬆️

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