Adding missing constraint to type signature is broken when "forall" is present

[jhrcek]

Reproduced with
haskell-language-server version: 0.8.0.0 (GHC: 8.8.4) (PATH: /home/jhrcek/.local/bin/haskell-language-server) (GIT hash: bde552b)

NOTE: previously I've made this code action more robust by fixing the behavior when context is formatted over multiple lines (haskell/ghcide#839). But it seems some more work is needed. I'll probably get to fixing this over the weekend.