Before -fdefer-type-errors there we no insolubles
(because we'd have failed before then), but with -fdefer-type-errors
there can be.  The code is acutally a bit simpler: we just call
emitConstraints, and eliminate the bogus-looking emitWC from TcRnMonad.

There's a bit more tidying up to do, concerning the places we use
keepWanted, but I need to talk to Dimitrios about that.

Meanwhile this fixes Trac #7023