[dev.inline] cmd/internal/src: make Pos implementation abstract

Adjust cmd/compile accordingly.

This will make it easier to replace the underlying implementation.

Change-Id: I33645850bb18c839b24785b6222a9e028617addb
Reviewed-on: https://go-review.googlesource.com/34133
Reviewed-by: David Lazar <lazard@golang.org>

src/cmd/compile/internal/ssa/sparsemap.go

@@ -66,7 +66,7 @@ func (s *sparseMap) setBit(k ID, v uint) {
 		s.dense[i].val |= 1 << v
 		return
 	}
-	s.dense = append(s.dense, sparseEntry{k, 1 << v, 0})
+	s.dense = append(s.dense, sparseEntry{k, 1 << v, src.Pos{}})
 	s.sparse[k] = int32(len(s.dense)) - 1
 }