JVM_IR: do not compute stored file signature when not needed

compiler/ir/serialization.jvm/src/org/jetbrains/kotlin/backend/jvm/serialization/JvmIdSignatureDescriptor.kt

@@ -108,12 +108,15 @@ class JvmIdSignatureDescriptor(mangler: KotlinMangler.DescriptorMangler) : IdSig
             }
 
         private fun computeStoredFileSignature(descriptor: DeclarationDescriptorWithSource) {
+            // isTopLevelPrivate needs to be already set.
+            if (isTopLevelPrivate && externallyGivenFileSignature == null) {
                 storedFileSignature = IdSignature.FileSignature(
                     descriptor.source.containingFile,
                     descriptor.containingPackage() ?: FqName.ROOT,
                     descriptor.source.containingFile.name ?: "unknown"
                 )
             }
+        }
 
         var storedFileSignature: IdSignature.FileSignature? = null
     }