...about

  static ImplImageTree instance;

in ImplImageTree::get (vcl/source/gdi/impimagetree.cxx) due to

  StyleIconSet ImplImageTree::maIconSet;

(of type std::unordered_map<OUString, IconSet, OUStringHash>) due to

  IconCache ImplImageTree::IconSet::maIconCache;

(of type std::unordered_map<OUString, std::pair<bool, BitmapEx>, OUStringHash>),
reported at least when building against Mac libc++.  This is a false positive,
as ImplImageTree::shutDown cleans this up.

Change-Id: Idb1a99aa33f71286e57b31b7adb833e3ee8b5f5a