swift234

[AST] Use the short-form @available() syntax in the AST printer where possible.

Reapply r28734. Argyrios updated SourceKit dependencies on swift and clang libraries in
SourceKit r28765, so this should work now.

Change the AST printer to use the new short-form @available attribute syntax for
attributes with only introduction versions. So, for example, a declaration
annotated as:
  @available(iOS, introduced=8.0) @available(OSX, introduced=10.10)
  func foo()
will be printed as:
  @available(iOS 8.0, OSX 10.10, *) func foo()

We won't include the attribute on the short form if it has a deprecated or
obsoleted version; nor if it has a message, a rename, or marks unconditional
unavailability.

This commit has a corresponding change to the SourceKit tests.

rdar://problem/20982322

Swift SVN r28768