The existing code was somewhat convoluted and made several assumptions
about the encoding of position bases:

1) The position's base for a file contained a position whose base
   pointed to itself (which is true but an implementation detail
   of src.Pos).

2) Updating the position base for a line directive required finding
   the base of the most recent's base position.

This change simply stores the file's position base and keeps using it
directly for each line directive (instead of getting it from the most
recently updated base).

Change-Id: I4d80da513bededb636eab0ce53257fda73f0dbc0
Reviewed-on: https://go-review.googlesource.com/95736Reviewed-by: Matthew Dempsky <mdempsky@google.com>