Fix CA2021 false positive for generic class types

[danpere]

Fix for #7357.

#7183 changed the CastWillAlwaysFail(ITypeSymbol castFrom, ITypeSymbol castTo) to work with castFrom.OriginalDefinition/castTo.OriginalDefinition, eliding the generic type information in order to fix issues with interfaces, but that appears to have broken the check for class types. This reverts just the class type case to use the castFrom/castTo types passed in instead of the .OriginalDefinition. I'm not sure I quite understand why that's the only place this matters, but I tried a few variations with structs and interfaces and couldn't get any other false positives.

[danpere]

@dotnet-policy-service agree [company="Microsoft"]

[danpere]

@dotnet-policy-service agree company="Microsoft"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.50%. Comparing base (5435ba7) to head (f2eea04).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7488   +/-   ##
=======================================
  Coverage   96.50%   96.50%           
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  Files        1450     1450           
  Lines      347319   347362   +43     
  Branches    11409    11409           
=======================================
+ Hits       335184   335232   +48     
+ Misses       9244     9241    -3     
+ Partials     2891     2889    -2     

[JoeRobich]

Thanks @danpere! I make sure this gets back-ported to 9.0.1xx

[borland]

As per #7357 (comment) this became un-fixed in SDK 9.0.200