swift234

Remove vestigial comma-separated attribute list for declarations.

For example:

  @class_protocol, @objc

is now just:

  @class_protocol @objc

Once we removed attribute grouping in brackets this comma separation
became vestigial.  Doug and I discussed this and thought this
was a good simplification in the grammar.

This change still remains to be done for type attributes.

Swift SVN r15540