Tab completion for enums not working as expected

### Description

Tab completion seems not to work as I would expect for enums.

### Repro steps

Start with something like:

```f#
type Color = Red=0
```

Try to add another entry:

```f#
type Color = Red=0
| Green=1
```

Pressing tab leaves the text at the beginning of the buffer. Same for any other entries added afterwards.

### Expected behavior

I would expect the indentation to look like:

```f#
type Color = Red=0
| Green=1
| Blue=2
```

### Known workarounds

If I manually indent the second entry in the enum to the "correct" level, all subsequent entries will take the same level. I have not found a way to properly indent the second enum entry without manually inserting spaces.

### Related information

* OSX
* 25.3
* .NET Core 1.0.1