[clang] Revert "[Clang][Sema] Fix lookup of dependent operator= outside of complete-class contexts (#91498)" (PR #91620)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

Is it alright to partially revert commits in a single PR? If so, I'm just going to revert all the `operator=` related changes from #91498, #90999, and #90152. 

https://github.com/llvm/llvm-project/pull/91620