Fix StaticSingletonManager inline fast path under gcc

Summary: [Folly] Fix StaticSingletonManager inline fast path under gcc - code size for default-initialized `static std::atomic<...>` local is large, but when value-initialized is trivial.

Reviewed By: andriigrynenko

Differential Revision: D14018861

fbshipit-source-id: e767ea3e46c3e3317f7b60008bc77f64edb2dea0

folly/detail/StaticSingletonManager.h

@@ -34,7 +34,7 @@ class StaticSingletonManager {
  public:
   template <typename T, typename Tag>
   FOLLY_EXPORT FOLLY_ALWAYS_INLINE static T& create() {
-    static Cache cache;
+    static Cache cache{};
     auto const& key = typeid(TypeTuple<T, Tag>);
     auto const v = cache.load(std::memory_order_acquire);
     auto const p = FOLLY_LIKELY(!!v) ? v : create_(key, make<T>, cache);