Summary:
The `reifyType` function in `TcSplice` is carefully designed
to avoid reifying visible arguments to `TyConApp`s. However, the same
care was not given towards the `AppTy` case, which lead to #15792.

This patch changes to the `AppTy` case of `reifyType` so that it
consults the kind of the function type to determine which of the
argument types are invisible (and therefore should be dropped) during
reification. This required crafting a variant of `tyConArgFlags`,
which I dubbed `appTyArgFlags`, that accept an arbitrary function
`Type` instead of a `TyCon`.

Test Plan: make test TEST=T15792

Reviewers: goldfire, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, carter

GHC Trac Issues: #15792

Differential Revision: https://phabricator.haskell.org/D5252