swift234

*Start* importing more availability information from the SDK.

This is a WIP.  This patch includes:

- Adds version tuple information for 'introduced', 'deprecated',
  and 'obsoleted' to the 'availability' attribute.

- Add Clang importer support to import __attribute__((availability))
  version tuples into Swift as pieces of the 'availability'
  attribute.

- Add serialization support for the 'availability' attribute with
  this extra information.  This is not tested other than the
  tests currently passing.  This is not expected to be
  really exercised (with interesting versions) until
  parsing support is added for the version tuples.  However,
  existing @availability attributes in the test suite are being
  serialized, which should just include "empty" version information.

What's not in this patch:

- Parsing support in Swift for 'deprecated', 'introduced', or
  'obsoleted'.  All of this information is currently being pulled
  in from the Clang Importer.

- Warning support for using deprecated declarations based on the
  availability information and the minimum deployment target.

- Some harmony reconciling the 'IsUnavailable' field in
  AvailabilityAttr, which attempts to eagerly compute if something
  is unavailable so we don't have to replicate the checking logic
  elsewhere.  The idea is that when we either import availability
  information or lazily deserialize it we can compute whether or
  not something is conditionally unavailable or deprecated right
  there, and not have to have all clients within the frontend
  of the availability information need to pass the minimum
  deployment target.  Right now 'IsUnavailable' is also used
  to encode if the attribute represents unconditional unavailability,
  e.g. @availability(*, unavailable).

This patch, however, should contain enough information to start
looking at implementing weak linking support.

NOTE: the serialization of the attribute is a bit ugly.  I wasn't
certain if Jordan's serialization meta-programming supported
serializing values that decomposed into multiple values in a record,
so this ugly macro-based implementation is in place which compacts
all the version tuple information for an availability attribute
into a single record.

Swift SVN r19487