[clang] [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 (PR #84050)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@erichkeane This PR is in a more "ready" state now, if you'd like to take another look. The added overload of `LookupTemplateName` which has no `MemberOfUnknownSpecialization` parameter it more of an experimental change... it can be ignored. 

https://github.com/llvm/llvm-project/pull/84050