Fix MSVC build

Summary: MSVC was failing to resolve `AllocTraits` within this specific context.

Reviewed By: yfeldblum

Differential Revision: D8179354

fbshipit-source-id: 236094f704bc89ec81236e9a67054e2081590cf6

folly/container/detail/F14Table.h

@@ -836,7 +836,7 @@ class F14Table : public Policy {
   using Policy::kDefaultConstructIsNoexcept;
   using Policy::kSwapIsNoexcept;
 
-  using typename Policy::AllocTraits;
+  using AllocTraits = typename Policy::AllocTraits;
 
   using ByteAlloc = typename AllocTraits::template rebind_alloc<uint8_t>;
   using BytePtr = typename std::allocator_traits<ByteAlloc>::pointer;