Fix comments with continuation

Backgound: aog/919954 tried to handle
```
   second line
```

but did it incorrectly. The parser works correctly (so this change
reverts aog/919954), it returns multiline comment, but the serializer
converting the internal representation to Blueprint was not emitting
'//' on the lines after the first.

Test: treehugger
Bug: 127521510
Change-Id: I0257a8b3cc4ffcaa6bea44113ceba66bb99d7e43

androidmk/androidmk/androidmk.go

@@ -137,7 +137,14 @@ func ConvertFile(filename string, buffer *bytes.Buffer) (string, []error) {
 
 		switch x := node.(type) {
 		case *mkparser.Comment:
-			file.insertComment("//" + x.Comment)
+			// Split the comment on escaped newlines and then
+			// add each chunk separately.
+			chunks := strings.Split(x.Comment, "\\\n")
+			file.insertComment("//" + chunks[0])
+			for i := 1; i < len(chunks); i++ {
+				file.bpPos.Line++
+				file.insertComment("//" + chunks[i])
+			}
 		case *mkparser.Assignment:
 			handleAssignment(file, x, assignmentCond)
 		case *mkparser.Directive:

androidmk/androidmk/androidmk_test.go

@@ -1260,10 +1260,10 @@ prebuilt_firmware {
 		desc: "comment with ESC",
 		in: `
 # Comment line 1 \
-# Comment line 2
+ Comment line 2
 `,
 		expected: `
-// Comment line 1 \
+// Comment line 1
 // Comment line 2
 `,
 	},