Change WarningCache to subclass set

[carmocca]

What does this PR do?

Tiny change. Should be okay as we don't access warnings anywhere

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[codecov]

Codecov Report

Merging #7995 (a055375) into master (b71aa55) will decrease coverage by 5%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7995    +/-   ##
=======================================
- Coverage      92%     87%    -5%     
=======================================
  Files         207     207            
  Lines       13375   13375            
=======================================
- Hits        12245   11607   -638     
- Misses       1130    1768   +638