[FIR] Do not smartcast class delegation implemented properties

When a property is implemented via class delegation, it should be
considered unstable for smart-casting. This is because it is unknown
what kind of stability the underlying class delegate property has. It
could be a stable property, or it could be implemented via a custom
getter and unstable.

^KT-57417 Fixed

compiler/testData/diagnostics/tests/smartCasts/varnotnull/classDelegation.fir.kt

@@ -0,0 +1,11 @@
+// ISSUE: KT-57417
+
+interface HasProperty {
+    val property: Int?
+}
+
+class Test(delegate: HasProperty) : HasProperty by delegate
+
+fun test(a: Test) {
+    if (a.property != null) <!SMARTCAST_IMPOSSIBLE!>a.property<!> + 1
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/classDelegation.kt

@@ -0,0 +1,11 @@
+// ISSUE: KT-57417
+
+interface HasProperty {
+    val property: Int?
+}
+
+class Test(delegate: HasProperty) : HasProperty by delegate
+
+fun test(a: Test) {
+    if (a.property != null) <!DEBUG_INFO_SMARTCAST!>a.property<!> + 1
+}