SOLR-14381: Integer overflow in total count in grouping results

[chatman]

Integer overflow in total count in grouping results. Please note that this is a PR on branch_8x, since it is more complex than the master change. 8x needs backward and forward compatability, and hence Utils.intIfNotOverflown(long) is used here. In the master variant of this patch, that method wouldn't be there and long would be expected everywhere (including tests).