Add PrivateInFileEffectiveVisibility language feature (1.6)

compiler/testData/diagnostics/tests/exposed/privateInFileDisabled.fir.kt

@@ -0,0 +1,30 @@
+// !LANGUAGE: -PrivateInFileEffectiveVisibility
+
+class Public {
+    private open class NestedPrivate
+
+    fun <!EXPOSED_FUNCTION_RETURN_TYPE!>test1<!>() = NestedPrivate()
+    fun test2(<!EXPOSED_PARAMETER_TYPE!>p: NestedPrivate<!>) {}
+    fun <!EXPOSED_RECEIVER_TYPE!>NestedPrivate<!>.test3() {}
+    val <!EXPOSED_PROPERTY_TYPE!>test4<!> = NestedPrivate()
+    class Test5 : <!EXPOSED_SUPER_CLASS!>NestedPrivate<!>()
+}
+
+private class PrivateInFileClass {
+    private open class NestedPrivate
+
+    fun test1() = NestedPrivate()
+    fun test2(p: NestedPrivate) {}
+    fun NestedPrivate.test3() {}
+    val test4 = NestedPrivate()
+    class Test5 : NestedPrivate()
+}
+
+private interface PrivateInFile {
+    private class Private
+
+    fun expose() = Private()
+}
+
+// Exposes 'PrivateInFile$Private' via 'expose'
+class Derived : PrivateInFile