Avoid spurious tree checker warning for higher order type params

TreeCheckers is trying to find references to a) local types
(term owned) or b) to type parameters from a trees that are
not ancestors of the a) term or b) type param owner. Such
references are ill-scoped and suggest that a tree has been
transplanted without proper substitution.

However, this check failed to account for higher order type
parameters, as seen in the spurious warning emitted by:

    test/pending/pos/treecheckers/c5.scala.

Author: retronym

src/compiler/scala/tools/nsc/typechecker/TreeCheckers.scala

@@ -376,7 +376,7 @@ abstract class TreeCheckers extends Analyzer {
         val treeInfo = infoOf(tree)
         val treeTpe  = typeOf(tree)
 
-        def isOk(sym: Symbol) = treeSym hasTransOwner sym.safeOwner
+        def isOk(sym: Symbol) = treeSym hasTransOwner sym.enclosingSuchThat(x => !x.isTypeParameterOrSkolem) // account for higher order type params
         def isEligible(sym: Symbol) = (sym ne NoSymbol) && (
              sym.isTypeParameter
           || sym.isLocal