Don't invalidate iterators into ScMyStylesMap

by changing it from std::unordered_map (to which it had been change from
std::set in ecf456d0 "tdf#125254 Performance: A
spreadsheet opens too slow, part3") to std::map.  CppunitTest_sc_bugfix_test had
started to fail, at least when building against GCC trunk libstdc++ in debug
mode (see my comment at <https://gerrit.libreoffice.org/#/c/72514/> "tdf#125254
Performance: A spreadsheet opens too slow, part3" for details).

Change-Id: I70d667fd008145107642bf780dc4ffe6acda947e
Reviewed-on: https://gerrit.libreoffice.org/72582
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

sc/source/filter/xml/XMLStylesImportHelper.hxx

@@ -27,7 +27,7 @@
 #include <list>
 #include <memory>
 #include <set>
-#include <unordered_map>
+#include <map>
 #include <vector>
 #include <boost/optional.hpp>
 
@@ -107,7 +107,7 @@ public:
 };
 
 /** map from style name to ScMyStyleRanges */
-typedef std::unordered_map<OUString, std::unique_ptr<ScMyStyleRanges>>  ScMyStylesMap;
+typedef std::map<OUString, std::unique_ptr<ScMyStyleRanges>>  ScMyStylesMap;
 
 class ScMyStylesImportHelper
 {