Fix JetType.equals(): flexible types are not equal to non-flexible ones, when we store them in a HashSet

compiler/testData/diagnostics/tests/inference/immutableArrayList.kt

@@ -0,0 +1,17 @@
+// FILE: p/J.java
+package p;
+
+public interface J {
+    public interface Super<T> {}
+    public interface Sub<T> extends Super<T> {}
+}
+
+// FILE: k.kt
+
+import p.J.*
+
+class Foo<T>: Sub<T> {
+    fun foo(): Super<T> {
+        return <!TYPE_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>Foo<!>()
+    }
+}