Change enable_thread_stack_temporaries to an RAII class

Change enable_thread_stack_temporaries to an RAII class

This started as a patch to change enable_thread_stack_temporaries to
be an RAII class, but then I noticed that this code used a VEC, so I
went ahead and did a bit more C++-ification, changing
stack_temporaries_enabled to a bool and changing stack_temporaries to
a std::vector.

Regression tested by the buildbot.

gdb/ChangeLog
2018-03-08  Tom Tromey  <tom@tromey.com>

	* infcall.c (struct call_return_meta_info)
	<stack_temporaries_enabled>: Remove.
	(get_call_return_value, call_function_by_hand_dummy): Update.
	* thread.c (disable_thread_stack_temporaries): Remove.
	(enable_thread_stack_temporaries): Remove.
	(thread_stack_temporaries_enabled_p): Return bool.
	(push_thread_stack_temporary, value_in_thread_stack_temporaries)
	(get_last_thread_stack_temporary): Update.
	* eval.c (evaluate_subexp): Update.
	* gdbthread.h (class enable_thread_stack_temporaries): Now a
	class, not a function.
	(value_ptr, value_vec): Remove typedefs.
	(class thread_info) <stack_temporaries_enabled>: Now bool.
	<stack_temporaries>: Now a std::vector.
	(thread_stack_temporaries_enabled_p)
	(value_in_thread_stack_temporaries): Return bool.