Fix panic on [t| _ |] (Trac #9879)

Summary:
Type brackets containing a wildcard, e.g. `[t| _ |]`, caused a panic.
Fix it by disallowing wildcards in type brackets.

Together with D572, this fixes #9879.

Test Plan: new test WildcardInTypeBrackets should pass

Reviewers: austin

Reviewed By: austin

Subscribers: carter, thomie, monoidal

Differential Revision: https://phabricator.haskell.org/D573

GHC Trac Issues: #9879