Improve makefile parser

Improve the androidmk makefile parser based on ideas from go/ast and
   - Use type switching instead of the As* mess
   - Don't store endPos for every node, compute it based on the last
     known position in the node plus the length of the last token
   - Store positions as only the offset into the file, and then unpack
     them into Line/Column scanner.Position objects later

Change-Id: I87eb6661859951e6c2ea5a85db6229fa5561d615
7 files changed