Cannot specify Literal attribute in signature file

```
range.fs(203, 17): [FS0034] Module 'FSharp.Compiler.Range' contains    val UnknownFileName : string    but its signature specifies    val UnknownFileName : string    The literal constant values and/or attributes differ
```

Related StackOverflow topic:
https://stackoverflow.com/questions/48212721/unable-to-use-the-literal-attribute-in-an-f-signature-file-fsi