swift234

Start implementing support for varargs tuple shuffles (thus, support
for varargs).  This is incomplete, but we can already lower this:

  var va2 : (Int...) = (1,2,3)

to this CFG:

  %15 = tuple (%4, %9, %14)
  %16 = alloc_array element type=Int, 3
  %17 = tupleelement %16, 1
  %18 = tupleelement %15, 0
  %19 = store %18 -> %17 [initialization]
  %20 = index_lvalue %17, 1
  %21 = tupleelement %15, 1
  %22 = store %21 -> %20 [initialization]
  %23 = index_lvalue %17, 2
  %24 = tupleelement %15, 2
  %25 = store %24 -> %23 [initialization]

Next, we need to call the injection function to turn an initialized heap 
allocation into Slice<T>... which requires a bunch of generic support.

To get this far, this patch:
- Introduces index_lvalue for indexing over array elements (striding lvalues)
- Introduces alloc_array.
- Renames RequalifyInst to TypeConversionInst and generalizes it to handle
  FunctionConversionExpr.



Swift SVN r2988