Use putAttachment in exportForwarders to handle ambiguous overloads

exportForwarders can be called more than once for the same expression
if there are ambiguous overloads.

Closes #21071

Author: wjoel

Diff for: compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1452,10 +1452,11 @@ class Namer { typer: Typer =>
           forwarders.derivedCons(forwarder2, avoidClashes(forwarders2))
         case Nil => forwarders
 
-      addForwarders(selectors, Nil)
-      val forwarders = avoidClashes(buf.toList)
-      exp.putAttachment(ExportForwarders, forwarders)
-      forwarders
+      exp.getAttachment(ExportForwarders).getOrElse:
+        addForwarders(selectors, Nil)
+        val forwarders = avoidClashes(buf.toList)
+        exp.pushAttachment(ExportForwarders, forwarders)
+        forwarders
     end exportForwarders
 
     /** Add forwarders as required by the export statements in this class */