Fix false positive for UnusedPrivateClass on generics

[cortinico]

Covered the scenario for more complex type parameters. I've also added a couple of tests.

Fixes #2791

[codecov]

Codecov Report

Merging #2793 into master will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #2793      +/-   ##
============================================
- Coverage     80.53%   80.51%   -0.02%     
  Complexity     2324     2324              
============================================
  Files           386      386              
  Lines          6955     6955              
  Branches       1261     1262       +1     
============================================
- Hits           5601     5600       -1     
  Misses          725      725              
- Partials        629      630       +1     

Impacted Files	Coverage Δ	Complexity Δ
...rturbosch/detekt/rules/style/UnusedPrivateClass.kt	67.21% <50.00%> (-1.64%)	4.00 <0.00> (ø)

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