[Threading] Create new threading library and use it.

Moved all the threading code to one place.  Added explicit support for
Darwin, Linux, Pthreads, C11 threads and Win32 threads, including new
implementations of Once for Linux, Pthreads, C11 and Win32.

rdar://90776105

include/swift/Runtime/Concurrent.h

@@ -21,7 +21,7 @@
 #include "llvm/Support/Allocator.h"
 #include "Atomic.h"
 #include "Debug.h"
-#include "Mutex.h"
+#include "swift/Threading/Mutex.h"
 
 #if defined(__FreeBSD__) || defined(__CYGWIN__) || defined(__HAIKU__)
 #include <stdio.h>
@@ -624,7 +624,7 @@ using llvm::hash_value;
 /// process. It has no destructor, to avoid generating useless global destructor
 /// calls. The memory it allocates can be freed by calling clear() with no
 /// outstanding readers, but this won't destroy the static mutex it uses.
-template <class ElemTy, class MutexTy = StaticMutex>
+template <class ElemTy, class MutexTy = LazyMutex>
 struct ConcurrentReadableHashMap {
   // We don't call destructors. Make sure the elements will put up with this.
   static_assert(std::is_trivially_destructible<ElemTy>::value,
@@ -1171,7 +1171,7 @@ template <class ElemTy> struct HashMapElementWrapper {
 /// by allocating them separately and storing pointers to them. The elements of
 /// the hash table are instances of HashMapElementWrapper. A new getOrInsert
 /// method is provided that directly returns the stable element pointer.
-template <class ElemTy, class Allocator, class MutexTy = StaticMutex>
+template <class ElemTy, class Allocator, class MutexTy = LazyMutex>
 struct StableAddressConcurrentReadableHashMap
     : public ConcurrentReadableHashMap<HashMapElementWrapper<ElemTy>, MutexTy> {
   // Implicitly trivial destructor.