Fix SingletonThreadLocal use from headers

Summary:
[Folly] Fix `SingletonThreadLocal` use from headers included by separate dynamically-loaded modules.

There is no need in `SingletonThreadLocal` specifically to check for double-registration and `SingletonThreadLocal` does not support mocks, so `LeakySingleton` may be bypassed entirely.

Reviewed By: djwatson

Differential Revision: D7283390

fbshipit-source-id: 45295c4dab233b9373ed32a47d4c80832b0f9eff

folly/SingletonThreadLocal.h

@@ -97,8 +97,9 @@ class SingletonThreadLocal {
   };
 
   FOLLY_EXPORT FOLLY_ALWAYS_INLINE static Wrapper& getWrapperInline() {
-    static LeakySingleton<ThreadLocal<Wrapper>, Tag> singleton;
-    return *singleton.get();
+    /* library-local */ static auto entry =
+        detail::createGlobal<ThreadLocal<Wrapper>, Tag>();
+    return **entry;
   }
 
   FOLLY_NOINLINE static Wrapper& getWrapperOutline() {