Initializer is now required for mutable properties with backing fields and open or custom setter #KT-9449 Fixed

Setters without body are not taken into account accordingly to KT-9449.
Old INITIALIZATION_USING_BACKING_FIELD_SETTER are both dropped.

compiler/testData/diagnostics/tests/backingField/InitCustomSetter.kt

@@ -0,0 +1,30 @@
+class My(val v: Int) {
+    // Ok: setter is just private
+    var x: Int
+        private set
+
+    <!MUST_BE_INITIALIZED!>var y: Int<!>
+        set(arg) { field = arg }
+
+    <!MUST_BE_INITIALIZED!>var z: Int<!>
+        set(arg) { field = arg }
+
+    // Ok: initializer available
+    var w: Int = v
+        set(arg) { field = arg }
+
+    // Ok: no backing field
+    var u: Int
+        get() = w
+        set(arg) { w = 2 * arg }
+
+    constructor(): this(0) {
+        z = v        
+    }
+
+    init {
+        x = 1
+        y = 2
+        u = 3
+    }
+}