[lldb][plugin] Clear in same thread as set

Here we were initializing & locking a mutex in a thread, while releasing it in the parent which may/often turned out to be a different thread (shared_mutex::unlock_shared is undefined behavior if called from a thread that doesn't hold the lock).

I'm not quite sure what the expectation is here as the variable is never used, so instead I've just reset in same thread as which it was set to ensure its freed in thread holding lock.

Author: jpienaar

lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.cpp

@@ -121,6 +121,7 @@ void ManualDWARFIndex::Index() {
       units_to_index.size());
   for_each_unit([&clear_cu_dies](size_t, size_t idx, DWARFUnit *unit) {
     clear_cu_dies[idx] = unit->ExtractDIEsScoped();
+    ckear_cu_duex[idx].reset();
   });
 
   // Now index all DWARF unit in parallel.