speed up SwDoc::IsInHeaderFooter() by using SwFrmFmtAnchorMap

Now that it's possible to quickly find anchored objects for a node,
it's actually faster to check the node hiearchy rather than layout.

Change-Id: I5f93d5af32c744f1773535e5ec8537334d1ff58a
avoid unused argument warning

Change-Id: Ib67b804423b521407710f8319cb89977e44494cc