String interpolation bug fix

This change fixes a bug in class StringContext.scala.
Parts were not correctly added to the resulting string.

This commit includes a test case which covers the
example reported in the bug.

Closes SI-5614.

Author: dgruntz

src/library/scala/StringContext.scala

@@ -144,7 +144,7 @@ object StringContext {
     var cur = 0
     var idx = 0
     def output(ch: Char) = {
-      bldr append str substring (start, cur)
+      bldr append str.substring (start, cur)
       bldr append ch
       start = idx
     }

test/files/run/t5614.check

@@ -0,0 +1,3 @@
+3
+a
+b

test/files/run/t5614.flags

@@ -0,0 +1 @@
+-Xexperimental

test/files/run/t5614.scala

@@ -0,0 +1,5 @@
+object Test extends App {
+  val str = s"a\nb"
+  println(str.length)
+  println(str)
+}