Fixing modality in fake overrides

The modality was nondeterministic before.
The main change sits in OverrideResolver: the logic of "fake override" generation is restructured so that
all the descriptors a newly created "fake" one overrides are known by the time it is created, so its
modality can be determined properly from their modalities.

Also, the ReadJavaBinaryClassTestGenerated is now a clear JUnit3 test case

compiler/testData/diagnostics/tests/override/InvisiblePotentialOverride.kt

@@ -0,0 +1,7 @@
+open class A {
+    private fun foo() : Int = 1
+}
+
+class B : A() {
+    fun foo() : String = ""
+}