Issue Description:

If you deprecate something with the unconstrained:

@available(*, deprecated)

then later change your mind and decide to tie it to a Swift version, it's easy to accidentally write this:

@available(*, deprecated: 4.2)

which will keep the deprecation firing regardless of Swift version. You probably meant this:

@available(swift, deprecated: 4.2)

Worse, if you introduce a second availability:

@available(*, deprecated, obsoleted: 4.2)

this doesn't obsolete and it no longer warns.

Supplying versions alongside * should probably trigger a warning, similar to this one:

@available(swift, deprecated)
Warning: Unknown platform 'swift' for attribute 'available'

This warning could probably also be reworded to be clearer, since swift is a totally known platform, it just needs a version.