Add missing #include <cassert>

[tautschnig]

cassert is implicitly provided via nonstd/optional, but this will change
with the move to std::optional. The even better fix would be the
replacement of all uses of assert(...) in these files, but this is a
larger endeavour.

• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6757 (1828a14) into develop (c0f179c) will increase coverage by 0.02%.
The diff coverage is 86.88%.

@@             Coverage Diff             @@
##           develop    #6757      +/-   ##
===========================================
+ Coverage    76.87%   76.89%   +0.02%     
===========================================
  Files         1590     1590              
  Lines       183887   183977      +90     
===========================================
+ Hits        141364   141475     +111     
+ Misses       42523    42502      -21     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_expr.cpp	77.06% <ø> (ø)
src/ansi-c/c_typecheck_initializer.cpp	76.49% <ø> (ø)
src/ansi-c/c_typecheck_type.cpp	77.08% <ø> (ø)
src/ansi-c/goto_check_c.cpp	91.47% <ø> (-0.15%)	⬇️
src/cpp/cpp_instantiate_template.cpp	79.16% <ø> (ø)
src/cpp/cpp_typecheck.cpp	79.22% <ø> (ø)
src/cpp/cpp_typecheck_code.cpp	80.59% <ø> (ø)
src/cpp/cpp_typecheck_compound_type.cpp	86.84% <ø> (ø)
src/cpp/cpp_typecheck_constructor.cpp	89.10% <ø> (ø)
src/cpp/cpp_typecheck_conversions.cpp	85.14% <ø> (ø)
... and 25 more

---

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[tautschnig]

Closing as #6754 will now take care of all remaining assert statements.