De-duplicate explicit-allocation check construction

[tautschnig]

We had almost-the-same code in two places. Use the method introduced in
cfd387d as the single implementation.

• 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 #5852 (50f775e) into develop (4d7158f) will increase coverage by 0.00%.
The diff coverage is 93.47%.

@@           Coverage Diff            @@
##           develop    #5852   +/-   ##
========================================
  Coverage    72.87%   72.88%           
========================================
  Files         1421     1421           
  Lines       154208   154256   +48     
========================================
+ Hits        112386   112428   +42     
- Misses       41822    41828    +6     

Impacted Files	Coverage Δ
src/util/format_expr.cpp	85.77% <92.40%> (+0.77%)	⬆️
src/analyses/goto_check.cpp	88.21% <100.00%> (-0.05%)	⬇️

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[NlightNFotis]

Good stuff 👍🏻