[FIR2IR] Create delegated field for delegation to var property

^KT-65920 Fixed

compiler/testData/codegen/box/delegation/delegationToVar.kt

@@ -0,0 +1,20 @@
+// ISSUE: KT-65920
+
+interface I {
+    fun foo()
+}
+
+class Test2(var j: I) : I by j
+
+fun box(): String {
+    var result = ""
+
+    val x = Test2(object : I { override fun foo() { result += "1" }})
+    x.foo()
+    x.j = object : I { override fun foo() { result += "2" }}
+    x.foo()
+    return when (result) {
+        "11" -> "OK"
+        else -> "Fail: $result"
+    }
+}