[tautschnig]

Per the GCC documentation the builtin returns its first argument and lets the compiler assume the returned pointer's alignment (with an optional misalignment offset in the variadic part). Model the identity value flow; do not assume or assert the alignment claim, which keeps the model conservative for verification.

Previously any call yielded "no body for callee
__builtin_assume_aligned" and a nondet return value, which poisoned every pointer flowing through libc++'s
std::__1::__assume_aligned/__add_alignment_assumption chain (used throughout vector's allocation path; seen in the
libcxx23_vector_pushback KNOWNBUG carrier).

The regression/cbmc-library test is required by the ansi-c target's library-check completeness gate and is included in this commit so every tree state builds.

• 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.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.03%. Comparing base (826c1c7) to head (2bcba21).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9431   +/-   ##
========================================
  Coverage    81.02%   81.03%           
========================================
  Files         1723     1723           
  Lines       191314   191314           
  Branches        73       73           
========================================
+ Hits        155020   155023    +3     
+ Misses       36294    36291    -3     

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