goto-analyzer: support typecasts as left-hand sides

[tautschnig]

This arises with aarch64 va_list structs (but possibly also in other cases).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.36%. Comparing base (0bb2d02) to head (29421f3).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8659   +/-   ##
========================================
  Coverage    80.36%   80.36%           
========================================
  Files         1686     1686           
  Lines       206984   206989    +5     
  Branches        73       73           
========================================
+ Hits        166333   166338    +5     
  Misses       40651    40651           

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[martin-cs]

I think this fixes the issue. I'm not 100% sure that it won't cause type mismatches when working down the write stack but I think these will hopefully be caught in the eval code.