Add explicit lowering for value packs and pack expansions.

- SILPackType carries whether the elements are stored directly
  in the pack, which we're not currently using in the lowering,
  but it's probably something we'll want in the final ABI.
  Having this also makes it clear that we're doing the right
  thing with substitution and element lowering.  I also toyed
  with making this a scalar type, which made it necessary in
  various places, although eventually I pulled back to the
  design where we always use packs as addresses.

- Pack boundaries are a core ABI concept, so the lowering has
  to wrap parameter pack expansions up as packs.  There are huge
  unimplemented holes here where the abstraction pattern will
  need to tell us how many elements to gather into the pack,
  but a naive approach is good enough to get things off the
  ground.

- Pack conventions are related to the existing parameter and
  result conventions, but they're different on enough grounds
  that they deserve to be separated.

lib/SILOptimizer/Utils/CastOptimizer.cpp

@@ -677,6 +677,9 @@ CastOptimizer::optimizeBridgedSwiftToObjCCast(SILDynamicCastInst dynamicCast) {
       break;
     }
     break;
+  case ParameterConvention::Pack_Guaranteed:
+  case ParameterConvention::Pack_Owned:
+  case ParameterConvention::Pack_Inout:
   case ParameterConvention::Direct_Owned:
   case ParameterConvention::Indirect_In:
     // Currently this