[lldb] Don't read firstSubclass and nextSiblingClass from class_rw_t (llvm#171213)

We're considering modifying the ObjC runtime's class_rw_t structure to
remove the firstSubclass and nextSiblingClass fields in some cases. LLDB
is currently reading those but not actually using them. Stop doing that
to avoid issues if they are removed by the runtime.

rdar://166084122
(cherry picked from commit 0c0ed39)

Author: JDevlieghere

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp

@@ -111,26 +111,23 @@ bool ClassDescriptorV2::class_rw_t::Read(Process *process, lldb::addr_t addr) {
                           process->GetAddressByteSize());
 
   lldb::offset_t cursor = 0;
-
   m_flags = extractor.GetU32_unchecked(&cursor);
   m_version = extractor.GetU32_unchecked(&cursor);
   m_ro_ptr = extractor.GetAddress_unchecked(&cursor);
   if (ABISP abi_sp = process->GetABI())
     m_ro_ptr = abi_sp->FixCodeAddress(m_ro_ptr);
   m_method_list_ptr = extractor.GetAddress_unchecked(&cursor);
   m_properties_ptr = extractor.GetAddress_unchecked(&cursor);
-  m_firstSubclass = extractor.GetAddress_unchecked(&cursor);
-  m_nextSiblingClass = extractor.GetAddress_unchecked(&cursor);
 
   if (m_ro_ptr & 1) {
     DataBufferHeap buffer(ptr_size, '\0');
     process->ReadMemory(m_ro_ptr ^ 1, buffer.GetBytes(), ptr_size, error);
     if (error.Fail())
       return false;
-    cursor = 0;
     DataExtractor extractor(buffer.GetBytes(), ptr_size,
                             process->GetByteOrder(),
                             process->GetAddressByteSize());
+    lldb::offset_t cursor = 0;
     m_ro_ptr = extractor.GetAddress_unchecked(&cursor);
     if (ABISP abi_sp = process->GetABI())
       m_ro_ptr = abi_sp->FixCodeAddress(m_ro_ptr);

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h

@@ -133,9 +133,6 @@ class ClassDescriptorV2 : public ObjCLanguageRuntime::ClassDescriptor {
     lldb::addr_t m_properties_ptr;
     lldb::addr_t m_protocols_ptr;
 
-    ObjCLanguageRuntime::ObjCISA m_firstSubclass;
-    ObjCLanguageRuntime::ObjCISA m_nextSiblingClass;
-
     bool Read(Process *process, lldb::addr_t addr);
   };