[GHC] #9628: Add Annotations to the AST to simplify source to source conversions

[GHC]

#9628: Add Annotations to the AST to simplify source to source conversions
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              Reporter:  alanz       |            Owner:  alanz
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:  D297        |
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Comment (by alanz):

 I have updated ​Phab:D426, ​Phab:D438 and ​Phab:D412 to include
 documentation about what annotations can be expected against each AST
 element, and also rebases against current master.

 The process of documentation showed up a number of shortcomings where
 annotations were missing or duplicated.

 I have also changed the `getAnnotation` result from `Maybe SrcSpan` to
 `[SrcSpan]` and allow multiple annotations of the same type on a given AST
 element. This means that the horrible `AnnColon2` and similar are now
 gone.

 The users of the annotations should be able to work out what is what from
 the surrounding context.

 They are once more ready for final review and hopefully merge for 7.10

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9628#comment:39>
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