...which apparently didn't take into account that inserting into a
std::unordered_map may invalidate iterators, which now started to cause build
failures like

> [PRP] dictionaries/hu_HU/dialog/hu_HU_de
> warn:sal.osl:25175:25175:sal/osl/unx/thread.cxx:1026: RTL_TEXTENCODING_DONTKNOW -> _ASCII_US
> .../gcc/trunk/inst/include/c++/9.0.1/debug/safe_iterator.h:307:
> In function:
>     __gnu_debug::_Safe_iterator<_Iterator, _Sequence, _Category>::pointer
>     __gnu_debug::_Safe_iterator<_Iterator, _Sequence,
>     _Category>::operator->() const [with _Iterator =
>     std::__detail::_Node_iterator<std::pair<const rtl::OString,
>     std::unique_ptr<MergeData> >, false, true>; _Sequence =
>     std::__debug::unordered_map<rtl::OString, std::unique_ptr<MergeData> >;
>     _Category = std::forward_iterator_tag;
>     __gnu_debug::_Safe_iterator<_Iterator, _Sequence, _Category>::pointer =
>     std::pair<const rtl::OString, std::unique_ptr<MergeData> >*]
>
> Error: attempt to dereference a singular iterator.
>
> Objects involved in the operation:
>     iterator "this" @ 0x0x5a8228 {
>       type = std::__detail::_Node_iterator<std::pair<rtl::OString const, std::unique_ptr<MergeData, std::default_delete<MergeData> > >, false, true> (mutable iterator);
>       state = singular;
>       references sequence with type 'std::__debug::unordered_map<rtl::OString, std::unique_ptr<MergeData, std::default_delete<MergeData> >, std::hash<rtl::OString>, std::equal_to<rtl::OString>, std::allocator<std::pair<rtl::OString const, std::unique_ptr<MergeData, std::default_delete<MergeData> > > > >' @ 0x0x5a81c8
>     }
> /bin/sh: line 1: 25175 Aborted                 (core dumped) LD_LIBRARY_PATH=${LD_LIBRARY_PATH:+$LD_LIBRARY_PATH:}"$I/program:$I/program" $W/LinkTarget/Executable/propex -i $S/dictionaries/hu_HU/dialog/hu_HU_en_US.properties -l de -m ${MERGEINPUT} -o $W/PropertiesTranslateTarget/dictionaries/hu_HU/dialog/hu_HU_de.properties
> make[1]: *** [.../solenv/gbuild/Dictionary.mk:88: .../workdir/PropertiesTranslateTarget/dictionaries/hu_HU/dialog/hu_HU_de.properties] Error 134

with trunk libstdc++ in debug mode.

Both classes MergeData and MergeDataHashMap became redundant.

Change-Id: I771548a6155e14037d84acfd74cd13566a26a8d7
Reviewed-on: https://gerrit.libreoffice.org/69395
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>