KTIJ-20329 "Redundant 'inner' modifier" may break code in case of type parameters on outer class

[t-kameyama]

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https://youtrack.jetbrains.com/issue/KTIJ-20329

[nikitabobko]

Please, run the inspection on the entire intellij-community project (how to run the inspection on the entire project) and either fix all the warnings introduced by the inspection (in separate commit but in this PR) or share your results as a comment.