Avoid too eager transform of $outer for lhs & accessor rhs (#18949)

Fix #18927 

The transformer in `mapOuter` in `Constructors` was transforming trees
it should not:
```scala
   override def transform(tree: Tree)(using Context) = tree match {
        [....]
        case tree: RefTree if tree.symbol.is(ParamAccessor) && tree.symbol.name == nme.OUTER =>
          ref(outerParam)
       [...]
 ```
There were two problems:
 - This case transformed LHS of `$outer` assignments in constructors. So, instead of setting the `$outer` field in the current class with the constructor, it was replaced with the $outer of the outer class. That resulted in not assigning any value to the `$outer` in inner class, and double assignment to the val in outer class.
 - LHS of the accessor def was also transformed, so it was evaluated to the `$outer` of the outer class.

This only happened when the nested class is created in the secondary constructor, as the primary constructor is not transformed (only the template body)

compiler/src/dotty/tools/dotc/transform/Constructors.scala

@@ -15,6 +15,7 @@ import Symbols.*
 import Decorators.*
 import DenotTransformers.*
 import collection.mutable
+import Types.*
 
 object Constructors {
   val name: String = "constructors"
@@ -197,6 +198,10 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
              ) &&
              fn.symbol.info.resultType.classSymbol == outerParam.info.classSymbol =>
           ref(outerParam)
+        case Assign(lhs, rhs) if lhs.symbol.name == nme.OUTER => // not transform LHS of assignment to $outer field
+            cpy.Assign(tree)(lhs, super.transform(rhs))
+        case dd: DefDef if dd.name.endsWith(nme.OUTER.asSimpleName) => // not transform RHS of outer accessor
+          dd
         case tree: RefTree if tree.symbol.is(ParamAccessor) && tree.symbol.name == nme.OUTER =>
           ref(outerParam)
         case _ =>

tests/pos/i18927.scala

@@ -0,0 +1,14 @@
+class A
+
+class B {
+  val a = new A
+
+  class C(i: Int) {
+    def this() = {
+      this(1)
+      class Inner() {
+        println(a)
+      }
+    }
+  }
+}