[Concurrency][Threading] Remove use of platform thread functions.

The concurrency library can use the new threading library functions,
which avoids the problem of including <windows.h>.

rdar://90776105

include/swift/Threading/Thread.h

@@ -25,11 +25,15 @@ namespace swift {
 
 /// Identifies a thread
 class Thread {
+public:
+  using Id = threading_impl::thread_id;
+
 private:
-  threading_impl::thread_id id_;
+  Id id_;
 
 public:
   Thread() {}
+  explicit Thread(Id platformId) : id_(platformId) {}
   Thread(const Thread& other) : id_(other.id_) {}
   Thread(Thread&& other) : id_(std::move(other.id_)) {}
 
@@ -43,6 +47,9 @@ public:
     return *this;
   }
 
+  /// Returns the platform specific thread ID
+  Id platformThreadId() const { return id_; }
+
   /// Returns the currently executing thread
   static Thread current() {
     return Thread(threading_impl::thread_get_current());