[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/ModuleFile.cpp

@@ -1356,6 +1356,15 @@ Type ModuleFile::getType(TypeID TID) {
     break;
   }
 
+  case decls_block::ARRAY_TYPE: {
+    TypeID baseID;
+    uint64_t size;
+    decls_block::ArrayTypeLayout::readRecord(scratch, baseID, size);
+
+    typeOrOffset = ArrayType::get(getType(baseID), size, ctx);
+    break;
+  }
+
   default:
     // We don't know how to deserialize this kind of type.
     error();