Allow as-patterns in pattern synonym declarations.
We can allow them if they contain no free variables. This patch just allows them in one direction and not to be used as builders as the original ticket suggests. Test Plan: ./validate Reviewers: austin, bgamari Reviewed By: bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1666 GHC Trac Issues: #9739 Conflicts: testsuite/tests/patsyn/should_fail/all.T
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