Integrate nullability into cone types, add & use FIR flexible type

FIR fake overrides are rendered now more precisely to test this process

idea/testData/fir/multiModule/basicWithJavaFakeOverride/jvm/simpleFakeOverride.kt

@@ -0,0 +1,9 @@
+
+class Some
+
+class B : A<Some>() {
+    fun test() {
+        foo(Some())
+    }
+}
+