[Backport] Fix CA2021 false positive for generic class types

[JoeRobich]

Backport of #7488

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.49%. Comparing base (3d61c57) to head (f9a1058).
Report is 3 commits behind head on release/9.0.1xx.

Additional details and impacted files

@@               Coverage Diff                @@
##           release/9.0.1xx    #7493   +/-   ##
================================================
  Coverage            96.49%   96.49%           
================================================
  Files                 1443     1443           
  Lines               345885   345928   +43     
  Branches             11374    11374           
================================================
+ Hits                333765   333810   +45     
+ Misses                9238     9236    -2     
  Partials              2882     2882