[GHC] #12105: merge MatchFixity and HsMatchContext

[GHC]

#12105: merge MatchFixity and HsMatchContext
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           Reporter:  alanz          |             Owner:  alanz
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 `MatchFixity` was introduced to facilitate use of API Annotations.

 `HsMatchContext` does the same thing with more detail, but is chased
 through all over the place to provide context when processing a `Match`.

 Since we already have `MatchFixity` in the AST, it may as well provide the
 full context.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12105>
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