stdlib: use more idiomatic closure syntax

Author: gribozavr

stdlib/private/StdlibUnittest/RaceTest.swift

@@ -501,17 +501,17 @@ public func runRaceTest<RT : RaceTestWithPerTrialDataType>(
   let racingThreadCount = threads ?? max(2, _stdlib_getHardwareConcurrency())
   let sharedState = _RaceTestSharedState<RT>(racingThreadCount: racingThreadCount)
 
-  let masterThreadBody: (_: ()) -> () = {
-    (_: ()) -> () in
+  let masterThreadBody: () -> Void = {
+    () -> Void in
     for _ in 0..<trials {
       autoreleasepool {
         _masterThreadOneTrial(sharedState)
       }
     }
   }
 
-  let racingThreadBody: (Int) -> () = {
-    (tid: Int) -> () in
+  let racingThreadBody: (Int) -> Void = {
+    (tid: Int) -> Void in
     for _ in 0..<trials {
       _workerThreadOneTrial(tid, sharedState)
     }