Make LinearizeGraph() public (and move it to nodes files)

Rationale:
It is strange that HLinearOrderIterator is defined (and visible)
in nodes.h, but clients have no way to build this order. This CL
makes the building available at the usual place.

Change-Id: Ib66f2edf6dfc8edd6b429bd4bea3ac7e37440b28
Tests: m test-art