Add outermost utility method to SourcePosition.

Not needed right now, but might be useful in the future.

Author: odersky

compiler/src/dotty/tools/dotc/util/SourcePosition.scala

@@ -61,6 +61,9 @@ extends interfaces.SourcePosition with Showable {
   def focus   : SourcePosition = withSpan(span.focus)
   def toSynthetic: SourcePosition = withSpan(span.toSynthetic)
 
+  def outermost: SourcePosition =
+    if outer == null || outer == NoSourcePosition then this else outer.outermost
+
   override def toString: String =
     s"${if (source.exists) source.file.toString else "(no source)"}:$span"