Fir2Ir: create file level signatures where appropriate

This is godugly code, where a flag for file level signatures is passsed
around.
An alternative would be not to create file level signatures for toplevel
private clases, since those still need unique names, at least on JVM.
But that would break binary compatibility.
Signatures are due for overhaul anyway. Hopefully this code can be
reverted at that point.

native/native.tests/tests-gen/org/jetbrains/kotlin/konan/blackboxtest/NativeCodegenBoxTestGenerated.java

@@ -42100,6 +42100,12 @@ public class NativeCodegenBoxTestGenerated extends AbstractNativeCodegenBoxTest
                 runTest("compiler/testData/codegen/boxInline/private/nestedInPrivateClass.kt");
             }
 
+            @Test
+            @TestMetadata("nestedInPrivateClass2.kt")
+            public void testNestedInPrivateClass2() throws Exception {
+                runTest("compiler/testData/codegen/boxInline/private/nestedInPrivateClass2.kt");
+            }
+
             @Test
             @TestMetadata("privateClass.kt")
             public void testPrivateClass() throws Exception {