Add ConstThreadState to mach_extensions.h and use it everywhere

client/simulate_crash_mac.cc

Index: client/simulate_crash_mac.cc
diff --git a/client/simulate_crash_mac.cc b/client/simulate_crash_mac.cc
index 053b50a26d730c08515a7602a08d4bdeb3982baa..cdcf662d68ed16e7707b0a32a74cd0b420b1bae8 100644
--- a/client/simulate_crash_mac.cc
+++ b/client/simulate_crash_mac.cc
@@ -70,9 +70,6 @@ bool DeliverException(thread_t thread,
     set_state = false;
   }
 
-  // A const version of thread_state_t.
-  using ConstThreadState = const natural_t*;
-
   // old_state is only used if the context already captured doesn’t match (or
   // can’t be converted to) what’s registered for the handler.
   thread_state_data_t old_state;