Add unchecked marks for technically unsafe type tests

`Tree` here is `tpd.Tree` and we're checking against
`tpd.Tree | (Context => tpd.Tree)`. Because `Function1` is a trait, we
cannot rule out that there exists some subclass of `Tree` which extends
it and is not a `tpd.Tree`.

The unchecked warning started appearing after the first commit in this PR.

Author: smarter

compiler/src/dotty/tools/dotc/core/Annotations.scala

@@ -86,7 +86,7 @@ object Annotations {
       myTree.asInstanceOf[Tree]
 
     override def isEvaluating: Boolean = myTree == null
-    override def isEvaluated: Boolean = myTree.isInstanceOf[Tree]
+    override def isEvaluated: Boolean = myTree.isInstanceOf[Tree @unchecked]
   }
 
   /** An annotation indicating the body of a right-hand side,
@@ -119,7 +119,7 @@ object Annotations {
       myTree.asInstanceOf[Tree]
 
     override def isEvaluating: Boolean = myTree == null
-    override def isEvaluated: Boolean = myTree.isInstanceOf[Tree]
+    override def isEvaluated: Boolean = myTree.isInstanceOf[Tree @unchecked]
   }
 
   object LazyBodyAnnotation {