c++: use auto_timevar instead of timevar_push/pop

r12-5487-g9bf69a8558638c replaced uses of timevar_cond_push/pop with
auto_cond_timevar and removed now unnecessary wrapper functions.  This
patch does the same with timevar_push/pop and auto_timevar.

gcc/cp/ChangeLog:

	* parser.cc: Use auto_timevar instead of timevar_push/pop.
	Remove wrapper functions.
	* pt.cc: Likewise.