When grouping pattern synonyms in the desugarer, to find when a single
match will work for the whole group, we use `Match.sameGroup`.  But this
function was declaring two pattern-synonym matches equal to often.
Result: Lint errors and broken semantics.

The fix is easy.  See Note [Pattern synonym groups].

Re-do typechecking for pattern synonym signatures

Test Plan: Validate

Reviewers: austin

Subscribers: thomie, mpickering, simonpj

Differential Revision: https://phabricator.haskell.org/D1684