swift234

Add '*' wildcard to #os()

On platforms that are not explicitly mentioned in the #os() guard, this new '*'
availability check generates a version comparison against the minimum deployment target.

This construct, based on feedback from API review, is designed to ease porting
to new platforms. Because new platforms typically branch from
existing platforms, the wildcard allows an API availability check to do the "right"
thing (executing the guarded branch accessing newer APIs) on the new platform without
requiring a modification to every availability guard in the program.

So, if the programmer writes:

  if #os(OSX >= 10.10, *) {
  . . .
  }

and then ports the code to iOS, the body will execute.

We still do compile-time availability checking with '*', so the compiler will
emit errors for references to potentially unavailable symbols in the body when compiled
for iOS.

We require a '*' clause on all #os() guards to force developers to
"future proof" their availability checks against the introduction of new a platform.

Swift SVN r26988