This fixes #15471
In the typechecker we check that the splice has the right type but we
crucially don't zonk the generated expression. This is because we might
end up unifying type variables from outer scopes later on.
Reviewers: simonpj, goldfire, bgamari
Subscribers: rwbarton, carter
GHC Trac Issues: #15471
Differential Revision: https://phabricator.haskell.org/D5286