[LLDB] Reapply #100443 SBSaveCore Thread list (#104497)

Reapply #100443 and #101770. These were originally reverted due to a
test failure and an MSAN failure. I changed the test attribute to
restrict to x86 (following the other existing tests). I could not
reproduce the test or the MSAN failure and no repo steps were provided.

lldb/source/API/SBThread.cpp

@@ -1337,6 +1337,8 @@ bool SBThread::SafeToCallFunctions() {
   return true;
 }
 
+lldb::ThreadSP SBThread::GetSP() const { return m_opaque_sp->GetThreadSP(); }
+
 lldb_private::Thread *SBThread::operator->() {
   return get();
 }