Fix incorrect "self reference in refinement is deprecated" warning

The check used to disallow any reference of the form `X.this.type` even though
`X` is never the refinement class.

This change mirrors the check done with `ThisType` a couple of line below.

Author: smarter

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -412,7 +412,7 @@ object Checking {
         case tree: RefTree =>
           checkRef(tree, tree.symbol)
           foldOver(x, tree)
-        case tree: This =>
+        case tree: This if tree.tpe.classSymbol == refineCls =>
           selfRef(tree)
         case tree: TypeTree =>
           val checkType = new TypeAccumulator[Unit] {

tests/neg-custom-args/fatal-warnings/refinements-this.scala

@@ -0,0 +1,3 @@
+class Outer:
+  type X = { type O = Outer.this.type } // ok
+  type Y = { type O = this.type } // error

tests/pos/polymorphic-functions-this.scala

@@ -4,3 +4,7 @@ trait Foo:
   val f: [T <: this.X] => (T, this.X) => (T, this.X) =
     [T <: this.X] => (x: T, y: this.X) => (x, y)
   f(x, x)
+
+  val g: [T <: this.type] => (T, this.type) => (T, this.type) =
+    [T <: this.type] => (x: T, y: this.type) => (x, y)
+  g(this, this)