[GHC] #11218: Provide `-W(no-)` aliases for `-f(no-)warn` warning flags

[GHC]

#11218: Provide `-W(no-)` aliases for `-f(no-)warn` warning flags
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        Reporter:  hvr               |                Owner:
            Type:  feature request   |               Status:  patch
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1613
       Wiki Page:  Design/Warnings   |
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Description changed by bgamari:

Old description:

> TODO: description

New description:

 Move warning options out of the `-f` namespace and into `-W` where they
 belong. See [[Design/Warnings]] for details.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11218#comment:2>
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