The position information recorded now consists of the line-
directive relative filename and line number. It would be
relatively easy to also encode absolute position information
as necessary (by serializing src.PosBase data).

For example, given $GOROOT/src/tmp/x.go:

	package p

	const C0 = 0

	//line c.go:10
	const C1 = 1

	//line t.go:20
	type T int

	//line v.go:30
	var V T

	//line f.go:40
	func F() {}

The recorded positions for the exported entities are:

	C0 $GOROOT/src/tmp/x.go 3
	C1 c.go 10
	T t.go 20
	V v.go 30
	F f.go 40

Fix verified by manual inspection. There's currently no easy way
to test this, but it will eventually be tested when we fix #7311.

Fixes #19391.

Change-Id: I6269067ea58358250fe6dd1f73bdf9e5d2adfe3d
Reviewed-on: https://go-review.googlesource.com/37936Reviewed-by: Matthew Dempsky <mdempsky@google.com>