[GHC] #10692: Replace (SourceText, FastString) with WithSourceText data type

[GHC]

#10692: Replace (SourceText,FastString) with WithSourceText data type
-------------------------------------+-------------------------------------
              Reporter:  alanz       |             Owner:  alanz
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Phab:D907 introduced `SourceText` for a number of data types, by replacing
 `FastString` with `(SourceText,FastString)`. Since this has an
 `Outputable` instance, no warnings are generated when `ppr` is called on
 it, but unexpected output is generated. See Phab:D1096 for an example of
 this.

 Replace the `(SourceText,FastString)` tuples with a new data type

 {{{#!hs
 data WithSourceText = WithSourceText SourceText FastString
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10692>
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