[clang-tidy] Added a case to UnconventionalAssignOperatorCheck.

Summary:
The check accepts now a `return (*this = something);` as return
statement too (beneath of `*this`).

Reviewers: alexfh, hokein, aaron.ballman, JonasToth

Reviewed By: aaron.ballman

Subscribers: xazax.hun, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D74529