swift234

Start serializing out DeclAttribute objects using generalized serialization logic.

To generalize our serialization logic for more attributes, serialize
each DeclAttribute object in a separate bitcode record.

For simple declaration attributes (no arguments), all of this
serialization logic can be fully automatically generated, and is
done so in this patch.  This currently includes @final, but will
expand over time.

To illustrate the plumbing end-to-end, move the serialization logic
for asmnmame over to the new mechanism.

Swift SVN r15933