[nfc] [lldb] Simplify calling SymbolFileDWARF::GetDWARFCompileUnit

[jankratochvil]

Summary

Only SymbolFileDWARF::ParseCompileUnit creates a CompileUnit and it uses DWARFCompileUnit for that.

[labath]

Makes sense.

lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.h

protected:
   uint32_t CalculateNumCompileUnits() override;
 
   lldb::CompUnitSP ParseCompileUnitAtIndex(uint32_t index) override;
 
   lldb_private::TypeList &GetTypeList() override;
 
   lldb::CompUnitSP ParseCompileUnit(DWARFCompileUnit &dwarf_cu);
 
-  virtual DWARFUnit *
+  virtual DWARFCompileUnit *
   GetDWARFCompileUnit(lldb_private::CompileUnit *comp_unit);
 
   DWARFUnit *GetNextUnparsedDWARFCompileUnit(DWARFUnit *prev_cu);
 
   bool GetFunction(const DWARFDIE &die, lldb_private::SymbolContext &sc);
 
   lldb_private::Function *ParseFunction(lldb_private::CompileUnit &comp_unit,
                                         const DWARFDIE &die);

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

llvm::call_once(m_info_once_flag, [&] {
     static Timer::Category func_cat(LLVM_PRETTY_FUNCTION);
     Timer scoped_timer(func_cat, "%s this = %p", LLVM_PRETTY_FUNCTION,
                        static_cast<void *>(this));
     m_info = std::make_unique<DWARFDebugInfo>(*this, m_context);
   });
   return *m_info;
 }
 
-DWARFUnit *
-SymbolFileDWARF::GetDWARFCompileUnit(lldb_private::CompileUnit *comp_unit) {
+DWARFCompileUnit *SymbolFileDWARF::GetDWARFCompileUnit(CompileUnit *comp_unit) {
   if (!comp_unit)
     return nullptr;
 
   // The compile unit ID is the index of the DWARF unit.
   DWARFUnit *dwarf_cu = DebugInfo().GetUnitAtIndex(comp_unit->GetID());
   if (dwarf_cu && dwarf_cu->GetUserData() == nullptr)
     dwarf_cu->SetUserData(comp_unit);
-  return dwarf_cu;
+
+  // It must be DWARFCompileUnit when it created a CompileUnit.
+  return llvm::cast_or_null<DWARFCompileUnit>(dwarf_cu);
 }
 
 DWARFDebugRanges *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));
 
static uint64_t GetDWOId(DWARFCompileUnit &dwarf_cu,
   if (!dwo_id)
     dwo_id = cu_die.GetAttributeValueAsUnsigned(&dwarf_cu, DW_AT_dwo_id, 0);
   return dwo_id;
 }
 
 llvm::Optional<uint64_t> SymbolFileDWARF::GetDWOId() {
   if (GetNumCompileUnits() == 1) {
     if (auto comp_unit = GetCompileUnitAtIndex(0))
-      if (DWARFCompileUnit *cu = llvm::dyn_cast_or_null<DWARFCompileUnit>(
-              GetDWARFCompileUnit(comp_unit.get())))
+      if (DWARFCompileUnit *cu = GetDWARFCompileUnit(comp_unit.get()))
         if (DWARFDebugInfoEntry *cu_die = cu->DIE().GetDIE())
           if (uint64_t dwo_id = ::GetDWOId(*cu, *cu_die))
             return dwo_id;
   }
   return {};
 }
 
 std::shared_ptr<SymbolFileDWARFDwo>