Lift alloca's deallocation behaviour out of goto_check_ct

[tautschnig]

alloca always results in deallocation taking place, and not just when
pointer checks are enabled.

• 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 #6743 (da65bce) into develop (5c25144) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6743   +/-   ##
========================================
  Coverage    76.86%   76.86%           
========================================
  Files         1589     1589           
  Lines       183814   183829   +15     
========================================
+ Hits        141293   141308   +15     
  Misses       42521    42521           

Impacted Files	Coverage Δ
src/ansi-c/goto_check_c.cpp	91.47% <ø> (-0.15%)	⬇️
src/goto-programs/builtin_functions.cpp	58.35% <100.00%> (+1.48%)	⬆️
src/util/simplify_expr_int.cpp	85.59% <100.00%> (+0.09%)	⬆️

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