[lldb] Ensure that dwo/dwp are not double-indexed

DWO/DWP should not be indexed directly.
Instead, the corresponding base file should be used.
This diff adds an assert to DWARFCompileUnit::Index
and adjusts the methods 
SymbolFileDWARF::FindCompleteObjCDefinitionTypeForDIE,
SymbolFileDWARF::GetObjCMethodDIEOffsets accordingly.

Differential revision: https://reviews.llvm.org/D39825

llvm-svn: 318554
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
index ea2a11b..d7e13ed 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
@@ -209,6 +209,10 @@
   return nullptr;
 }
 
+DWARFCompileUnit *SymbolFileDWARF::GetBaseCompileUnit() {
+  return nullptr;
+}
+
 void SymbolFileDWARF::Initialize() {
   LogChannelDWARF::Initialize();
   PluginManager::RegisterPlugin(GetPluginNameStatic(),