[GHC] #12822: Cleanup GHC verbosity flags

[GHC]

#12822: Cleanup GHC verbosity flags
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           Reporter:  hsyl20         |             Owner:
               Type:  task           |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  newcomer,flags                     |
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 1) In the discussion of Phab:D2679#77898, Simon Marlow suggested that we
 make verbosity levels (`-v0`, `-v1`...) work just like optimization levels
 (`-O0`, `-O1`...), i.e., that we make each verbosity level a collection of
 verbosity flags.

 Currently the verbosity level is tested against explicitly in several
 places in GHC's code.

 2) Homogenize verbosity flags: currently, flags that change GHC's
 reporting have various prefixes:
   - `-fprint-*`
   - `-fshow-*`
   - `-dshow-passes`
   - `-ddump-*`
   - `-dppr-*`
   - `-dverbose-*`
 Maybe we should homogenize (some of) them by using a common `-v` prefix as
 in `-vshow-source-paths`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12822>
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