Try to workaround "a pure expression does nothing" warning

Ref scala/bug#12112

Current app transformation macro creates a tree that looks like:

```scala
FullInstance.app[[T0[x]](T0[Int], T0[Int]), Unit](scala.Tuple2(task2, task1), (($p$macro$3: (Int, Int)) => {
  <synthetic> val $q$macro$2: Int = $p$macro$3._1;
  <synthetic> val $q$macro$1: Int = $p$macro$3._2;
  {
    ($q$macro$1: Int);
    ($q$macro$2: Int);
    ()
  }
}))(AList.tuple2[Int, Int])
```

Starting Scala 2.12.12 the compiler's "pure expression does nothing" has become more enthusiastic/accurate in its reach, and it started warning about the naked reference to `$q$macro$1` that appears to do nothing, even though in reality it would trigger the tasks and do something in the context of sbt. It's just _that_ particular line ends up macroed away into a pure expression.

A somewhat bizarre workaround is to make a fake call to a method just to satisfy this warning. I've chosen `scala.Predef.identity` here so it can be composed together with the existing expression nesting when they exist.

Author: eed3si9n

core-macros/src/main/scala/sbt/internal/util/appmacro/Instance.scala

@@ -187,7 +187,8 @@ object Instance {
       qual.foreach(checkQual)
       val vd = util.freshValDef(tpe, qual.pos, functionSym)
       inputs ::= new Input(tpe, qual, vd)
-      util.refVal(selection, vd)
+      // try to workaround https://github.com/scala/bug/issues/12112 by calling Predef.identity(...)
+      q"scala.Predef.identity[$tpe](${vd.name}: $tpe)"
     }
     def sub(name: String, tpe: Type, qual: Tree, replace: Tree): Converted[c.type] = {
       val tag = c.WeakTypeTag[T](tpe)

sbt/src/sbt-test/project/setting-macro/project/PureExpressionPlugin.scala

@@ -0,0 +1,15 @@
+package pkgtest
+
+import sbt._, Keys._
+
+// https://github.com/scala/bug/issues/12112
+object PureExpressionPlugin extends AutoPlugin {
+  lazy val testPureExpression = taskKey[Unit]("")
+  override def projectSettings: Seq[Setting[_]] = {
+    testPureExpression := {
+      updateFull.value
+      (Compile / compile).value
+      (Test / test).value
+    }
+  }
+}

sbt/src/sbt-test/project/setting-macro/project/plugins.sbt

@@ -0,0 +1 @@
+Compile / scalacOptions += "-Xfatal-warnings"