Add tests for compilation against abi classes

plugins/jvm-abi-gen/testData/compile/privateOnlyConstructors/lib/classes.kt

@@ -0,0 +1,7 @@
+package lib
+
+class A private constructor(val x: Int) {
+    companion object {
+        fun create(x: Int): A = A(x * 2)
+    }
+}