Incremental SMT2 back-end: support _Bool casts

[tautschnig]

Added an extra case in extension_for_type to handle conversion between C boolean types and C integers by using zero extension.

Fixes: #8069

• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.93%. Comparing base (cdee49c) to head (56b9799).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8730   +/-   ##
========================================
  Coverage    79.93%   79.93%           
========================================
  Files         1698     1698           
  Lines       187698   187700    +2     
  Branches        73       73           
========================================
+ Hits        150034   150041    +7     
+ Misses       37664    37659    -5     

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