Pass #1 at localizing assumptions about fixed layout and

handling non-fixed layouts.

This uncovered a bug where we weren't rounding up the header
size to the element alignment when allocating an array of archetypes.

Writing up a detailed test case for *that* revealed that we were
never initializing the length field of heap arrays.  Fixing that
caused a bunch of tests to crash trying to release stuff.  So...
I've left this in a workaround state right now because I have to
catch a plane.

Swift SVN r4804

lib/IRGen/GenStruct.cpp

@@ -51,8 +51,8 @@ namespace {
   };
 
   /// Layout information for struct types.
-  class StructTypeInfo :
-    public SequentialTypeInfo<StructTypeInfo, StructFieldInfo> {
+  class StructTypeInfo : // FIXME: FixedTypeInfo as the base class is a lie.
+    public SequentialTypeInfo<StructTypeInfo, FixedTypeInfo, StructFieldInfo> {
   public:
     StructTypeInfo(llvm::Type *T, unsigned numFields)
       : SequentialTypeInfo(T, numFields) {