Allow comparisons between TypeRefs and ThisTypes.

Motivation comes from Linker:

List$.canBuildFrom is implemented as

  ReusableCBF.asInstanceOf[GenericCanBuildFrom[A]]

static type of field ReusableCBF here is
  TypeRef(TypeRef(List$), GenericCanBuildFrom[Nothing])
but the ident ReusableCBF gets type
  TypeRef(ThisType(List$), GenericCanBuildFrom[Nothing])

We already permit comparisons between static TermRefs and ThisTypes,
this change additionally allows TypeRefs to static modules to
become subtypes of ThisTypes of static modules.

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -218,7 +218,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
             val cls1 = tp1.cls
             cls1.classInfo.selfType.derivesFrom(cls2) &&
             cls2.classInfo.selfType.derivesFrom(cls1)
-          case tp1: TermRef if cls2.is(Module) && cls2.eq(tp1.widen.typeSymbol) =>
+          case tp1: NamedType if cls2.is(Module) && cls2.eq(tp1.widen.typeSymbol) =>
             cls2.isStaticOwner ||
             isSubType(tp1.prefix, cls2.owner.thisType) ||
             secondTry(tp1, tp2)