Language Service does not show FS0247 error

Using 16.0, but also reproduces in 15.9

1. Create a new project
2. Replace the contents of the first file with this:

```fsharp
namespace A

module B = begin end
```
3. Create a new file and replace its contents with this:

```fsharp
namespace A.B

type DU =
| X of float
| Y of float
```

Expected:

* Error in the error list and in the editor
* `FS0247 - A namespace and module named 'A.b' both occur in two parts of this assembly`

Actual:

* Nothing in the editor and nothing in the error list