...otherwise, at least an --enable-dbgutil --without-system-libxml build will
issue warnings at soffice.bin exit, as liblangtag merged together docs with
xmlCopyNode (instead of xmlDocCopyNode) and xmlAddChild, and libxml2's
debugXML.c xmlCtxtCheckName then stumbles over the merged-in nodes whose strings
do not belong to the dict of the doc they got merged into.

Change-Id: I5d7889670b897cbeb9ca8a555d8f2d770a8ee692