Make return type of dispatch_semaphore_create non-optional

Author: dgrove-oss

src/swift/Dispatch.swift

@@ -433,9 +433,8 @@ public func dispatch_group_leave(group:dispatch_group_t) -> Void {
 // semaphore.h
 //////////
 
-public func dispatch_semaphore_create(value:Int) -> dispatch_semaphore_t? {
-  let csem = CDispatch.dispatch_semaphore_create(value)
-  return csem != nil ? dispatch_semaphore_t(csem) : nil
+public func dispatch_semaphore_create(value:Int) -> dispatch_semaphore_t {
+  return dispatch_semaphore_t(CDispatch.dispatch_semaphore_create(value))
 }
 
 public func dispatch_semaphore_wait(dsema:dispatch_semaphore_t, _ timeout:dispatch_time_t) -> Int {