Do not allocate huge array in output_in_order.

Do not allocate huge array in output_in_order.

We noticed that when analyzing LTRANS memory peak that happens right
after the start:
https://gist.github.com/marxin/223890df4d8d8e490b6b2918b77dacad

In case of chrome, we have symtab->order == 200M, so we allocate
16B * 200M = 3.2GB.

gcc/ChangeLog:

	* cgraph.h: Remove leading empty lines.
	* cgraphunit.c (enum cgraph_order_sort_kind): Remove
	ORDER_UNDEFINED.
	(struct cgraph_order_sort): Add constructors.
	(cgraph_order_sort::process): New.
	(cgraph_order_cmp): New.
	(output_in_order): Simplify and push nodes to vector.

(cherry picked from commit 8bd062e8ad44e70be04108232e1ef597fc3b3e3e)