FIR: Adjust override checker to definitely-not-nullable types

^KT-52201 Related

compiler/testData/diagnostics/tests/j+k/overrideWithErasure.kt

@@ -0,0 +1,20 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+// FILE: Base.java
+public class Base<T extends CharSequence> {
+    public T foo(T t) { return t; }
+}
+
+// FILE: Derived.java
+public class Derived<E extends CharSequence> extends Base<E> {
+    @Override
+    public E foo(CharSequence e) {
+        return (E) "";
+    }
+}
+
+// FILE: main.kt
+
+fun main(d: Derived<CharSequence>) {
+    d.foo("")
+}