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

[Erich Keane via cfe-commits]

erichkeane 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 ?

Its sad to do so, but I think it makes the most sense.  We've got a good amount of feedback of the breakages, and I think figuring them out and reapplying these changes in a followup patch is a perfectly reasonable way forward.

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