cmExportSetMap: Override clear() to delete held resources

Replace the std::map<>::clear() method with one that first deletes
the cmExportSet instances held by each map entry, and then clears.
Otherwise the cmGlobalGenerator::ClearGeneratorMembers added by
commit 5cf1120f (cmGlobalGenerator: Refactor member cleanup between
configures, 2013-11-04) leaks the cmExportSet instances.
Merge topic 'clear-generator-data' into next

00055ce cmExportSetMap: Override clear() to delete held resources