swift234

Sema: Refine availability context in IfStmt guard conditions following #available()

Loosen restrictions on where #available() can appear in IfStmt guards and refine the
context for guard StmtConditionElements following an availability check.

This enables #available() to be combined with if let optional binding:

if #available(iOS 8.0, *),
   let x = someIOS8API() {
  // Do more iOS 8 stuff
}

and

if let x = someIOS7API() where #available(iOS 8.0, *),
   let y = someIOS8API() {
  // Do more iOS 8 stuff
}

Swift SVN r28096