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/A.java

@@ -0,0 +1,5 @@
+public class A<T> {
+    public T foo(T t) {
+        return t;
+    }
+}

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

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

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

@@ -0,0 +1,13 @@
+FILE: simpleFakeOverride.kt
+    public final class Some {
+        public constructor(): super<R|kotlin/Any|>()
+
+    }
+    public final class B : R|A<Some>| {
+        public constructor(): super<R|A<Some>|>()
+
+        public final function test(): R|kotlin/Unit| {
+            R|FakeOverride</A.foo: R|ft<T, T>|!>|(<Unresolved name: Some>#())
+        }
+
+    }