Two changes from Krasimir Angelov, which were required for Visual
Haskell:

  - messaging cleanup throughout the compiler.  DynFlags has a new
    field:

    log_action :: Severity -> SrcSpan -> PprStyle -> Message -> IO ()

    this action is invoked for every message generated by the
    compiler.  This means a client of the GHC API can direct messages to
    any destination, or collect them up in an IORef for later
    perusal.

    This replaces previous hacks to redirect messages in the GHC API
    (hence some changes to function types in GHC.hs).

  - The JustTypecheck mode of GHC now does what it says.  It doesn't
    run any of the compiler passes beyond the typechecker for each module,
    but does generate the ModIface in order that further modules can be
    typechecked.

And one change from me:

  - implement the LANGUAGE pragma, finally