Fix completion snippets on DuplicateRecordFields

[berberman]

Closes #682

According to GHC Wiki:

`data T = MkT { x :: Int }`

When DuplicateRecordFields is enabled, field accessors become "selectors". For example, the corresponding GlobalRdrElt of above x is x |-> GRE $sel:x:MkT (FldParent T (Just x)) LocalDef. So we can't use gre_name($sel:x:MkT) for completion, but par_lbl(x) of its parent.

In this PR, item names are converted into OccNames before passed to mkNameCompItem, resulting in no renamed filed selector occuring in the result.

• regression test

[pepeiborra]