Add multiplatform test with substitution, fix deep supertypes

This commit fixes ambiguity problems introduced before
in MPP tests with deep supertypes

#KT-29636 Fixed

idea/testData/fir/multiModule/mppFakeOverrides/common/common.kt

@@ -0,0 +1,10 @@
+expect open class A<T>() {
+    open fun foo(arg: T)
+}
+open class B : A<String>() {
+    // Fake: override fun foo(arg: String) = super.foo(arg)
+    // Fake (JVM only) (?): override fun bar(arg: String): String = super.bar(arg)
+}
+open class C : B() {
+    open fun bar(arg: CharSequence): String = arg.toString()
+}