Rename is bogus on generic parameters

```
type Generic<'t> = {
    T : 't
}
```

After invoking rename (Ctrl+R,R) on `t` or `'t` and entering `a` or `'a` respectively, the result is invalid code:
```
type Generic<``'a``> = {
    T : ``'a``
}
```

The expected result is
```
type Generic<'a> = {
    T : 'a
}
```

Visual Studio 15.7.5