[bug] wrong Type name for `type` in imenu-list when RankNTypes used

#### example
`Foo.hs`
``` hs
{-# LANGUAGE RankNTypes #-}
type Foo a = forall m . Monad m => m a
```

#### wrong result
`*Ilist* Foo.hs`
```
+ Datatypes
m
```

#### it should be
`*Ilist* Foo.hs`
```
+ Datatypes
Foo
```

Reproduce the issue with the RankNTypes example in Foo.hs and inspect the imenu-list type-name parsing entry point. Trace why the generated Datatypes entry is m rather than Foo, then verify that the imenu-list output uses Foo for the example.

Written by the indexing model from the issue text.