swift234

Add support for default initialized values with a new ZeroValue instruction. 

We can now compile:
  var defaulted_var : Int

into:

  %7 = alloc_var defaulted_var, type=[byref(heap)] Int
  %8 = zerovalueinst Int
  %9 = store %8 -> %7 [initialization]

It's questionable whether this is actually good language design (requiring
definitive assignment would be better) but the CFG needs this for IRGen parity.



Swift SVN r2972