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

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

should be fine to revert as much as you want in a single PR, just make sure to mention what you're reverting in the description

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