Add a const overload for ObjCInterfaceDecl::all_declared_ivar_begin.

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135741 91177308-0d34-0410-b5e6-96231b3b80d8