Improve makefile parser

Improve the androidmk makefile parser based on ideas from go/ast and
friends:
   - 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

androidmk/parser/make_strings.go

@@ -2,7 +2,6 @@ package parser
 
 import (
 	"strings"
-	"text/scanner"
 	"unicode"
 )
 
@@ -18,18 +17,30 @@ import (
 // of Variables.  The raw string list is always one longer than the variable
 // list.
 type MakeString struct {
-	Pos       scanner.Position
+	StringPos Pos
 	Strings   []string
 	Variables []Variable
 }
 
-func SimpleMakeString(s string, pos scanner.Position) *MakeString {
+func SimpleMakeString(s string, pos Pos) *MakeString {
 	return &MakeString{
-		Pos:     pos,
-		Strings: []string{s},
+		StringPos: pos,
+		Strings:   []string{s},
 	}
 }
 
+func (ms *MakeString) Pos() Pos {
+	return ms.StringPos
+}
+
+func (ms *MakeString) End() Pos {
+	pos := ms.StringPos
+	if len(ms.Strings) > 1 {
+		pos = ms.Variables[len(ms.Variables)-1].End()
+	}
+	return Pos(int(pos) + len(ms.Strings[len(ms.Strings)-1]))
+}
+
 func (ms *MakeString) appendString(s string) {
 	if len(ms.Strings) == 0 {
 		ms.Strings = []string{s}
@@ -97,7 +108,7 @@ func (ms *MakeString) Split(sep string) []*MakeString {
 func (ms *MakeString) SplitN(sep string, n int) []*MakeString {
 	ret := []*MakeString{}
 
-	curMs := SimpleMakeString("", ms.Pos)
+	curMs := SimpleMakeString("", ms.Pos())
 
 	var i int
 	var s string
@@ -115,7 +126,7 @@ func (ms *MakeString) SplitN(sep string, n int) []*MakeString {
 
 			for _, r := range split[1:] {
 				ret = append(ret, curMs)
-				curMs = SimpleMakeString(r, ms.Pos)
+				curMs = SimpleMakeString(r, ms.Pos())
 			}
 		} else {
 			curMs.appendString(s)
@@ -131,7 +142,9 @@ func (ms *MakeString) SplitN(sep string, n int) []*MakeString {
 }
 
 func (ms *MakeString) TrimLeftSpaces() {
+	l := len(ms.Strings[0])
 	ms.Strings[0] = strings.TrimLeftFunc(ms.Strings[0], unicode.IsSpace)
+	ms.StringPos += Pos(len(ms.Strings[0]) - l)
 }
 
 func (ms *MakeString) TrimRightSpaces() {