allow constructing a auto_vec with a preallocation, and a possibly larger actual...

allow constructing a auto_vec with a preallocation, and a possibly larger actual allocation size

This allows us to set the capacity of the vector when we construct it,
and still use a stack buffer when the size is small enough.

gcc/ChangeLog:

2017-05-13  Trevor Saunders  <tbsaunde+gcc@tbsaunde.org>

	* genrecog.c (int_set::int_set): Explicitly construct our
	auto_vec base class.
	* vec.h (auto_vec::auto_vec): New constructor.

From-SVN: r248019