Range of empty signature module expends to last comment

Please provide a succinct description of the issue.

Consider the following code in a signature file:

```fsharp
module foo

// bar
// baz
```

Matching AST:
```fsharp
SigFile
(ParsedSigFileInput
("tmp.fsi", QualifiedNameOfFile foo, [], [],
[SynModuleOrNamespaceSig
([foo], false, NamedModule, [],
PreXmlDoc ((1,6), FSharp.Compiler.XmlDoc+XmlDocCollector), [], None,
tmp.fsi (1,0--4,6) IsSynthetic=false)]))
```

**Expected behavior**

`SynModuleOrNamespaceSig` should span from line 1 to line 1.

**Actual behavior**

The last comment is included in the range of the `SynModuleOrNamespaceSig`.
This is not the case for the implementation file.

**Known workarounds**

Calculate the range using the name of the module/namespace.

**Related information**

FCS 39.