Add a const overload for ObjCInterfaceDecl::all_declared_ivar_begin.

This was previously not-const only because it has to lazily construct a chain
of ivars the first time it is called (and after the chain is invalidated).
In practice, all the clients were just const_casting their const Decls;
all those now-unnecessary const_casts have been removed.

llvm-svn: 135741