[DebugInfo] Add DWARFUnit::clearDWO()
Add DWARFUnit::clearDWO() so a skeleton unit can drop the DWO context
it owns without being destroyed itself. Also add DWARFUnit::hasDWO()
to answer if that skeleton CU is currently caching a DWO context, so
users can easily look it up.
For example, BOLT opened a DWARFContext for every .dwo during
readDebugInfo and kept them all alive until teardown. On large
split-DWARF targets that is tens of GiB held
resident. clearDWO()/hasDWO() expose to users DWARFUnit's caching
capacity, allowing them to spontaneously drop the cache/look it
up/re-load it for memory management.
