Kill varSetElems in tcInferPatSynDecl
Kill varSetElems in tcInferPatSynDecl

varSetElems introduces unnecessary non-determinism and while
I didn't estabilish experimentally that this matters here
I'm convinced that it will, because I expect pattern synonyms
to end up in interface files.

Test Plan: ./validate

Reviewers: austin, simonmar, bgamari, mpickering, simonpj

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2206

GHC Trac Issues: #4012

(cherry picked from commit 21fe4ffd049c8ab4b9ee36af3cf8f70b46d6beda)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc