Fix #6989: Make sure scalacLinkedClass finds companion when unpickling

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -801,6 +801,7 @@ class TreeUnpickler(reader: TastyReader,
           ValDef(tpt)
         case TYPEDEF | TYPEPARAM =>
           if (sym.isClass) {
+            sym.owner.ensureCompleted() // scalacLinkedClass uses unforcedDecls. Make sure it does not miss anything.
             val companion = sym.scalacLinkedClass
 
             // Is the companion defined in the same Tasty file as `sym`?

tests/pos/i6989/Container_1.scala

@@ -0,0 +1,12 @@
+package mypkg
+
+object Container {
+  class StringExtras(val s: String) extends AnyVal {
+    def op(item: Int): Int = ???
+  }
+}
+
+trait Container {
+  import Container._
+  implicit def mkStringExtras(s: String): StringExtras = new StringExtras(s)
+}

tests/pos/i6989/SimpleTest_2.scala

@@ -0,0 +1,5 @@
+package mypkg
+
+class SimpleTest extends Container {
+  "foo".op(5)
+}