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

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

Chromium is also seeing similar breakages. @sdkrystian is this breaking valid code? I can't tell from your latest comment. (if it is breaking valid code we should revert)

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