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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@aeubanks I think I'm going to revert this & maybe partially revert the changes in #90152 which cause `operator=` to be treated as a dependent name when the current class is templated. There are lots of edge cases that need to be accounted for. Thoughts @erichkeane ?

https://github.com/llvm/llvm-project/pull/91498