C front-end: reject pointer subtraction over distinct types

[tautschnig]

This is rejected by GCC, and we should equally refuse to process this.

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[codecov]

Codecov Report

Base: 78.40% // Head: 78.41% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (206cf2a) compared to base (540a60f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7441   +/-   ##
========================================
  Coverage    78.40%   78.41%           
========================================
  Files         1659     1659           
  Lines       190379   190382    +3     
========================================
+ Hits        149276   149280    +4     
+ Misses       41103    41102    -1     

Impacted Files	Coverage Δ
...cts/dynamic-frames/dfcc_lift_memory_predicates.cpp	89.50% <ø> (ø)
src/ansi-c/c_typecheck_expr.cpp	75.75% <100.00%> (+0.06%)	⬆️

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