tdf#122926 FILEOPEN ODT Slower compared to the past (part2)

Further improvement: on my machine, this takes the time from 8.9s to
5.7s

Change-Id: Ic7f7c46b10fbb671ff5bbf9d0a6a0195696741bc
Reviewed-on: https://gerrit.libreoffice.org/70112
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sw/source/core/txtnode/ndtxt.cxx

@@ -1394,7 +1394,7 @@ void SwTextNode::Update(
             bool bAtLeastOneBookmarkMoved = false;
             bool bAtLeastOneExpandedBookmarkAtInsertionPosition = false;
             // A text node already knows its marks via its SwIndexes.
-            std::unordered_set<const sw::mark::IMark*> aSeenMarks;
+            o3tl::sorted_vector<const sw::mark::IMark*> aSeenMarks;
             const SwIndex* next;
             for (const SwIndex* pIndex = GetFirstIndex(); pIndex; pIndex = next )
             {