timevar: Add auto_cond_timevar class

timevar: Add auto_cond_timevar class

The auto_timevar sentinel class for starting and stopping timevars was added
in 2014, but doesn't work for the many uses of timevar_cond_start/stop in
the C++ front end.  So let's add one that does.

This allows us to remove a lot of wrapper functions that were just used to
call timevar_cond_stop on all exits from the function.

gcc/ChangeLog:

	* timevar.h (class auto_cond_timevar): New.

gcc/cp/ChangeLog:

	* call.c
	* decl.c
	* name-lookup.c:
	Use auto_cond_timevar instead of timevar_cond_start/stop.
	Remove wrapper functions.