swift234

Remove @unavailable, and move to introducing basic (sham) parsing for @availability.

The parsing here for @availability isn't real yet; but it provides
scaffolding.  Intended grammar:

  @availability(*, unavailable, message="...")
  @availability(*, unavailable)

  @availability(ios, unavailable)

  and so on.

Much of this doesn't work yet in a general way, but I wanted something
basic to work with to start with to wire up the functionality
for @availability end-to-end (at least for 'unavailable').

As part of this, extend DECL_ATTR to include whether or not an
attribute supports multiple declarations, and use this for
@availability.

Also hardwire darwin platforms, which we will need to have this
list come from somewhere.  The checking could be done at parsing
or elsewhere.

Swift SVN r15491