Turn -fdefer-typed-holes on by default

From http://www.devalot.com/articles/2013/07/why-haskell.html:

You can, however, return an undefined value which when evaluated will terminate the program. Haskell programmers use this to stub out parts of the program during development and they’re easy to detect during a production build.↩

That's terrible, they should be using typed holes!

Hypothesis: because ghc -fdefer-typed-holes Test.hs is annoying to type, haskellers currently reach for undefined instead of typed holes (i.e. _), to mark the part of their code that isn't finished yet.

Richard writes in ticket:9497#comment:87391:

I'm -1 on [enabling -fdefer-typed-holes by default]. It seems to invite the possibility that holes could make their way into released code.

That's true, but the alternative is worse: developers keep using undefined instead of typed holes. Since ghc doesn't show any warnings for using undefined (#8064 (closed)), we should make it as easy as possible to use typed holes, which do show a warning (unless explicitly suppressed with -fno-warn-typed-holes). Having to specify a command-line flag is too high a barrier (since undefined is so well known and easy to use)

Proposal:

• ghc turns -fdefer-typed-holes on by default.
• build scripts specify -fno-defer-typed-holes to make sure no holes make it into releases.
• we encourage everyone to use _ instead of undefined.