DUChain support of defaulted/deleted functions again

[David Nolden]

Do we even need to track deleted declarations at all? Wouldn't it be
enough to simply not create them? Or is it possible to delete
explicity inherited declarations from parent classes?

Otherwise, this whole thing remembers me in AliasDeclaration, which is
created among other things by the "using ...;" directive. Maybe a
deleted declaration should simply be an AliasDeclaration which points
to some special "deleted" declaration or AbstractType or to no
declaration at all.

Greetings, David