[GHC] #11429: Make unrecognised `-W` flags a warning rather than an error

[GHC]

#11429: Make unrecognised `-W` flags a warning rather than an error
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        Reporter:  hvr               |                Owner:
            Type:  feature request   |               Status:  closed
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1830,
       Wiki Page:  Design/Warnings   |  Phab:D1942
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Comment (by thomie):

 This feature broke the alternative spelling suggestions for misspelled
 warning flags, see #11789.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11429#comment:12>
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