[serialization] Add support for ArrayTypes.

...in a quest for completeness. ArrayTypes don't actually work yet
(single-dimensional arrays are typed as slices), but when they do the test
that is currently XFAIL'd should start passing.

With this, all non-transient types can now be serialized and deserialized.

Swift SVN r6101

lib/Serialization/ModuleFormat.h

@@ -215,6 +215,7 @@ namespace decls_block {
     BOUND_GENERIC_SUBSTITUTION,
     POLYMORPHIC_FUNCTION_TYPE,
     ARRAY_SLICE_TYPE,
+    ARRAY_TYPE,
     REFERENCE_STORAGE_TYPE,
 
     TYPE_ALIAS_DECL = 100,
@@ -358,6 +359,12 @@ namespace decls_block {
     TypeIDField  // implementation type
   >;
 
+  using ArrayTypeLayout = BCRecordLayout<
+    ARRAY_TYPE,
+    TypeIDField, // element type
+    BCVBR<8>     // size
+  >;
+
   using ReferenceStorageTypeLayout = BCRecordLayout<
     REFERENCE_STORAGE_TYPE,
     BCFixed<1>,  // ownership