[clang][NFC] Break out BaseUsingDecl from UsingDecl

This is a pre-patch for adding using-enum support.  It breaks out
the shadow decl handling of UsingDecl to a new intermediate base
class, BaseUsingDecl, altering the decl hierarchy to

def BaseUsing : DeclNode<Named, "", 1>;
  def Using : DeclNode<BaseUsing>;
def UsingPack : DeclNode<Named>;
def UsingShadow : DeclNode<Named>;
  def ConstructorUsingShadow : DeclNode<UsingShadow>;

Differential Revision: https://reviews.llvm.org/D101777