Fix #3518: Do not generate calls to BoxesRunTime for Unit box/unbox

Author: nicolasstucki

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "scala-backend"]
 	path = scala-backend
-	url = https://github.com/lampepfl/scala.git
+	url = https://github.com/dotty-staging/scala.git
 	branch = sharing-backend
 [submodule "scala2-library"]
 	path = scala2-library

scala-backend

@@ -0,0 +1,4 @@
+()
+()
+A
+()

tests/run/i3518.check

@@ -0,0 +1,12 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    Unit.box(())
+    Unit.unbox(Unit.box(()))
+    println(Unit.box(()))
+    println(Unit.unbox(Unit.box(())))
+    println(Unit.box({
+      println("A")
+      ()
+    }))
+  }
+}