[clang] [clang-tools-extra] Reapply "[Clang][Sema] Diagnose class member access expressions naming non-existent members of the current instantiation prior to instantiation in the absence of dependent base classes (#84050)" (PR #90152)

[David Green via cfe-commits]

davemgreen wrote:

Hi - We've ran into a couple of places where this causes problems, one of them in running Spec as above. Is it possible to turn off this error for older codebases with a flag, turning it into a warning? It doesn't seem like a very useful error if it applies to code that is never used. https://godbolt.org/z/Pf83EW7vE

https://github.com/llvm/llvm-project/pull/90152