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

[Erich Keane via cfe-commits]

erichkeane 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

Agreed, a revert doesnt have to be a single commit, or even a whole one.  Just enough to get us back to a 'working' state is fine.

Though if you do 'partial' ones, I prefer to not REMOVE tests, but to put a FIXME on the tests you've reverted and fix their test lines (assuming they dont' crash)

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