resolved fdo#72288 made case insensitive matrix string query work again

The SharedString stored at ScQueryEntry::Item was constructed from an
OUString passed from ScMatrixValue::GetString() so
rItem.maString.getDataIgnoreCase() in QueryEvaluator::compareByString()
was NULL and never evaluated equal. Made ScMatrixValue::GetString()
return a SharedString instead.

Change-Id: I473d5724dfb97707fea58e6b72b1396c049b79c8