Fix unpickling an inner class in a Scala2 object without companion

When we unpickle a Scala2 module we might not have a companion class to unpickle,
when this happen we call markAbsent() on the denotation of the companion
class, which means that calls to companionModule will not work. Fixed by
falling back to the more robust scalacLinkedClass.

Author: smarter

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -827,7 +827,12 @@ class ClassfileParser(
         def getMember(sym: Symbol, name: Name)(implicit ctx: Context): Symbol =
           if (static)
             if (sym == classRoot.symbol) staticScope.lookup(name)
-            else sym.companionModule.info.member(name).symbol
+            else {
+              var module = sym.companionModule
+              if (module == NoSymbol && sym.isAbsent)
+                module = sym.scalacLinkedClass
+              module.info.member(name).symbol
+            }
           else
             if (sym == classRoot.symbol) instanceScope.lookup(name)
             else sym.info.member(name).symbol