Disable messages with GHC API

[Christopher Done]

In Intero, after loading modules, for each one I run the following
function: https://github.com/commercialhaskell/intero/blob/300ac5a/src/GhciInfo.hs#L75..L85

If there are warnings or any output, they get outputted. As they are
already outputted by regular :load, I don’t need the same output
twice.

How do I disable non-severe output for any GhcMonad m => m a? I’m
using GHC 8.0.1 presently.

I tried the following before calling getModInfo, expecting there to be
no output anymore:

+      GHC.setSessionDynFlags
+        df {log_action = \ref dflags severity srcSpan style msg -> return ()}

And this had no effect. I tried some other things but ran out of
patience to keep a record of them all.

Ciao!