Merge pull request scala#6463 from milessabin/topic/si-10779

Added test of literal type folding

Author: adriaanm

test/files/pos/t10779.scala

@@ -0,0 +1,7 @@
+object Test {
+  final val x: 1 = 1
+  final val y: 2 = 2
+  x + 2: 3 //ok
+  1 + y: 3 //ok
+  x + y: 3 //ok
+}