DWARF: Simplify SymbolFileDWARF::GetDWARFCompileUnit

[labath]

Summary

The DWARFCompileUnit is set as the "user data" of the lldb compile unit
directly in the constructor (see ParseCompileUnit).

This means that instead of going through unit indexes, we can just fetch
the DWARF unit directly from there.

[xiaobai]

Just a heads up: I reverted this change (rL362862) because it broke the test suite on macOS. Breakpoints were not being resolved correctly.

[labath]

Thanks for reverting this. It seems I did not take into account here the fact that SymbolFileDWARFDebugMap creates CompileUnits without setting the the UserData to point to the relevant dwarf compile unit.

I'll drop this patch and implement the thing I needed this for in D63005 differently.

source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp

const DWARFDebugInfo *SymbolFileDWARF::DebugInfo() const {
   return m_info.get();
 }
 
 DWARFUnit *
 SymbolFileDWARF::GetDWARFCompileUnit(lldb_private::CompileUnit *comp_unit) {
   if (!comp_unit)
     return nullptr;
 
-  DWARFDebugInfo *info = DebugInfo();
-  if (info) {
-    // The compile unit ID is the index of the DWARF unit.
-    DWARFUnit *dwarf_cu = info->GetUnitAtIndex(comp_unit->GetID());
-    if (dwarf_cu && dwarf_cu->GetUserData() == nullptr)
-      dwarf_cu->SetUserData(comp_unit);
-    return dwarf_cu;
-  }
-  return nullptr;
+  return static_cast<DWARFUnit *>(comp_unit->GetUserData());
 }
 
 DWARFDebugRangesBase *SymbolFileDWARF::GetDebugRanges() {
   if (!m_ranges) {
     static Timer::Category func_cat(LLVM_PRETTY_FUNCTION);
     Timer scoped_timer(func_cat, "%s this = %p", LLVM_PRETTY_FUNCTION,
                        static_cast<void *>(this));