"field": synthetic variable creation in accessors, codegen changed accordingly, a set of tests, relevant code fix (j2k)

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

@@ -0,0 +1,18 @@
+open class Base {
+    open val x: Int = 1
+        get() = field - 1
+}
+
+class Other: Base() {
+    override val x = 2
+}
+
+class Another: Base() {
+    override val x = 3
+        get() = field + 1
+}
+
+class NoBackingField: Base() {
+    override val x: Int
+        get() = 5
+}