[clang] [Clang][Sema] Fix lookup of dependent operator= named by using-declaration (PR #91503)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@pawosm-arm Could you provide some examples? Barring an issue with `operator=` which has been addressed, builds should only fail in the presence of accesses to non-existent members. Addressing such issues is the responsibility of users. 

https://github.com/llvm/llvm-project/pull/91503