Fix heap-use-after-free when clearing DIEs in fission compile units.

[rupprecht]

Summary

D131437 caused heap-use-after-free failures when testing TestCreateAfterAttach.py in asan mode, and "regular" crashes outside of asan.

This appears to be due to a mismatch in a couple places where we choose to clear the DIEs. When we clear the DIE of a skeleton unit, we unconditionally clear the DIE of the DWO unit if it exists. However, ~ScopedExtractDIEs() only looks at the skeleton unit when deciding to clear. If we decide to clear the skeleton unit because it is now unused, we end up clearing the DWO unit that _is_ used. This change adds a guard by checking m_cancel_scopes to prevent clearing the DWO unit.

This is 100% reproducible by running TestCreateAfterAttach.py in asan mode, although it only seems to reproduce in our internal build, so no test case is added here. If someone has suggestions on how to write one, I can add it.

[clayborg]

LGTM, but I will let Pavel comment since he did the fission support.

[clayborg]

Thanks for following up on this and fixing the issue!

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

dw_addr_t DWARFUnit::ReadAddressFromDebugAddrSection(uint32_t index) const {
   return LLDB_INVALID_ADDRESS;
 }
 
 // It may be called only with m_die_array_mutex held R/W.
 void DWARFUnit::ClearDIEsRWLocked() {
   m_die_array.clear();
   m_die_array.shrink_to_fit();
 
-  if (m_dwo)
+  if (m_dwo && !m_dwo->m_cancel_scopes)
     m_dwo->ClearDIEsRWLocked();
 }
 
 lldb::ByteOrder DWARFUnit::GetByteOrder() const {
   return m_dwarf.GetObjectFile()->GetByteOrder();
 }
 
 void DWARFUnit::SetBaseAddress(dw_addr_t base_addr) { m_base_addr = base_addr; }