Re-add qualifiedTypes in object language

Author: mbovel

library/src/scala/language.scala

@@ -293,6 +293,10 @@ object language {
     @compileTimeOnly("`separationChecking` can only be used at compile time in import statements")
     object separationChecking
 
+    /** Experimental support for qualified types */
+    @compileTimeOnly("`qualifiedTypes` is only be used at compile time")
+    object qualifiedTypes
+
     /** Experimental support for automatic conversions of arguments, without requiring
      *  a language import `import scala.language.implicitConversions`.
      *