stdlib: remove pthread_barrier attributes

These were not in use, and removing this from the signature makes it
possible to re-use the exact signature on Windows more easily.

Author: compnerd

Diff for: stdlib/private/SwiftPrivateThreadExtras/SwiftPrivateThreadExtras.swift

@@ -126,7 +126,7 @@ public class _stdlib_Barrier {
   public init(threadCount: Int) {
     self._pthreadBarrier = _stdlib_pthread_barrier_t()
     let ret = _stdlib_pthread_barrier_init(
-      _pthreadBarrierPtr, nil, CUnsignedInt(threadCount))
+      _pthreadBarrierPtr, CUnsignedInt(threadCount))
     if ret != 0 {
       fatalError("_stdlib_pthread_barrier_init() failed")
     }

Diff for: stdlib/private/SwiftPrivateThreadExtras/ThreadBarriers.swift

@@ -14,6 +14,8 @@
 import Darwin
 #elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
+#elseif os(Windows)
+import MSVCRT
 #endif
 
 //
@@ -22,22 +24,6 @@ import Glibc
 // (OS X does not implement them.)
 //
 
-public struct _stdlib_pthread_barrierattr_t {
-  public init() {}
-}
-
-public func _stdlib_pthread_barrierattr_init(
-  _ attr: UnsafeMutablePointer<_stdlib_pthread_barrierattr_t>
-) -> CInt {
-  return 0
-}
-
-public func _stdlib_pthread_barrierattr_destroy(
-  _ attr: UnsafeMutablePointer<_stdlib_pthread_barrierattr_t>
-) -> CInt {
-  return 0
-}
-
 public var _stdlib_PTHREAD_BARRIER_SERIAL_THREAD: CInt {
   return 1
 }
@@ -64,7 +50,6 @@ public struct _stdlib_pthread_barrier_t {
 
 public func _stdlib_pthread_barrier_init(
   _ barrier: UnsafeMutablePointer<_stdlib_pthread_barrier_t>,
-  _ attr: UnsafeMutablePointer<_stdlib_pthread_barrierattr_t>?,
   _ count: CUnsignedInt
 ) -> CInt {
   barrier.pointee = _stdlib_pthread_barrier_t()

Diff for: validation-test/stdlib/StringSlicesConcurrentAppend.swift

@@ -90,7 +90,7 @@ func sliceConcurrentAppendThread(_ tid: ThreadID) {
 StringTestSuite.test("SliceConcurrentAppend") {
   barrierVar = UnsafeMutablePointer.allocate(capacity: 1)
   barrierVar!.initialize(to: _stdlib_pthread_barrier_t())
-  var ret = _stdlib_pthread_barrier_init(barrierVar!, nil, 2)
+  var ret = _stdlib_pthread_barrier_init(barrierVar!, 2)
   expectEqual(0, ret)
 
   let (createRet1, tid1) = _stdlib_pthread_create_block(