UnusedPrivateClass: don't report if private classes are used for type conversion

[t-kameyama]

Fixes #6982

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.78%. Comparing base (a263ecb) to head (5762717).

Files	Patch %	Lines
...rturbosch/detekt/rules/style/UnusedPrivateClass.kt	50.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6995      +/-   ##
============================================
- Coverage     83.79%   83.78%   -0.02%     
  Complexity     3932     3932              
============================================
  Files           579      579              
  Lines         12100    12104       +4     
  Branches       2506     2508       +2     
============================================
+ Hits          10139    10141       +2     
  Misses          710      710              
- Partials       1251     1253       +2     

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[BraisGabin]

Thanks!

[detekt-ci]

	Warnings
⚠️	This PR is approved with no milestone set. If merged, it won't appear in the detekt release notes.

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