The SwFrmFmtAnchorMap class provides reverse mapping
to SwFrmFmt::GetAnchor().GetCntntAnchor(), so that when code somewhere needs
to update SwFrmFmt's anchored at a position, it's not necessary to iterate
over all of them (which can be a large number e.g. with mail merge).

One special catch with the multimap of SwNodeIndex keys is that
the values of those keys change (whenever the node structure of the document
changes, indexes of nodes change as a result). This makes it impossible to use
any hashing container, as the hashes would change without the container
noticing, but multimap should work fine, as it just requires that the keys
remain sorted, and that is the case. Nevertheless, the old code in the two
converted places is intentionally left there in debug mode to verify the reverse
mapping is updated correctly.

I intentionally went with SwNodeIndex rather than SwPosition, as SwIndex
(the other component of SwPosition) was causing some trouble (see e.g. the
SwPosition comparison operator< , where two same positions are different if one
is registered and the other not) and it doesn't appear to be actually necessary.

Change-Id: I7f1768558f60155d4ba83c84aa7f9e34dc65ebf9