[RFA] Change enable_thread_stack_temporaries to an RAII class

[Tom Tromey]

>> +  bool stack_temporaries_enabled;

Simon> I think we can just removed this field, it seems to be unused.

Thanks for noticing this, I've removed it.

Simon> The "if (!tp->stack_temporaries.empty ())" here is now unnecessary.

I made this change as well.

Tom