FIR: do not inherit external modifier from super declaration

compiler/testData/diagnostics/tests/external/noExternalModifierInheritance.kt

@@ -0,0 +1,10 @@
+// FIR_IDENTICAL
+open class A {
+    open external fun foo()
+}
+
+class B : A() {
+    override fun foo() {
+        super.foo()
+    }
+}