use thread_local instead of ThreadLocal for some statics in Random

Summary: Exit synchronization is the worst!  The worst!

Reviewed By: @​bmaurer

Differential Revision: D2253073

folly/Random.cpp

@@ -108,18 +108,13 @@ void BufferedRandomDevice::getSlow(unsigned char* data, size_t size) {
   ptr_ += size;
 }
 
-
 }  // namespace
 
 void Random::secureRandom(void* data, size_t size) {
-  static ThreadLocal<BufferedRandomDevice> bufferedRandomDevice;
-  bufferedRandomDevice->get(data, size);
+  static thread_local BufferedRandomDevice bufferedRandomDevice;
+  bufferedRandomDevice.get(data, size);
 }
 
-ThreadLocalPRNG::ThreadLocalPRNG() {
-  static folly::ThreadLocal<ThreadLocalPRNG::LocalInstancePRNG> localInstance;
-  local_ = localInstance.get();
-}
 
 class ThreadLocalPRNG::LocalInstancePRNG {
  public:
@@ -128,6 +123,11 @@ class ThreadLocalPRNG::LocalInstancePRNG {
   Random::DefaultGenerator rng;
 };
 
+ThreadLocalPRNG::ThreadLocalPRNG() {
+  static thread_local ThreadLocalPRNG::LocalInstancePRNG localInstance;
+  local_ = &localInstance;
+}
+
 uint32_t ThreadLocalPRNG::getImpl(LocalInstancePRNG* local) {
   return local->rng();
 }