Previously, we we only using `tcSplitSigmaTy` when determining if a
function had been applied to too few arguments, so it wouldn't work for
functions with nested `forall`s. Thankfully, this is easily fixed with a
dash of `tcSplitNestedSigmaTys`.

Test Plan: make test TEST=T13311

Reviewers: austin, bgamari, simonpj

Reviewed By: bgamari

Subscribers: goldfire, simonpj, rwbarton, thomie

GHC Trac Issues: #13311

Differential Revision: https://phabricator.haskell.org/D3678