Avoid errnoeous deprecation warning in F14BasicSet

Summary:
[Folly] Avoid errnoeous deprecation warning in `F14BasicSet`.

The deprecation warning is emitted for instantiations like `F14FastSet<std::string>`.

The warning is triggered by looking for move-constructibility of the mapped type for maps or `void` for sets, but `void` is not move-constructible. There are a variety of techniques to solve this, and the smallest change is to use `lift_unit_t` in the right place. `Unit` is move-constructible where `void` is not.

Reviewed By: nbronson

Differential Revision: D8178045

fbshipit-source-id: 1ca06e84b4ed7acd415901afcbe80663513063ea

folly/container/detail/F14Policy.h

@@ -21,6 +21,7 @@
 #include <type_traits>
 #include <utility>
 
+#include <folly/Unit.h>
 #include <folly/container/detail/F14Table.h>
 #include <folly/hash/Hash.h>
 #include <folly/lang/SafeAssert.h>
@@ -498,7 +499,7 @@ class ValueContainerPolicy : public BasePolicy<
 
   void moveItemDuringRehash(Item* itemAddr, Item& src) {
     complainUnlessNothrowMove<Key>();
-    complainUnlessNothrowMove<MappedTypeOrVoid>();
+    complainUnlessNothrowMove<lift_unit_t<MappedTypeOrVoid>>();
 
     constructValueAtItem(0, itemAddr, Super::moveValue(src));
     if (destroyItemOnClear()) {
@@ -1055,7 +1056,7 @@ class VectorContainerPolicy : public BasePolicy<
 
   void transfer(Alloc& a, Value* src, Value* dst, std::size_t n) {
     complainUnlessNothrowMove<Key>();
-    complainUnlessNothrowMove<MappedTypeOrVoid>();
+    complainUnlessNothrowMove<lift_unit_t<MappedTypeOrVoid>>();
 
     if (std::is_same<Alloc, std::allocator<Value>>::value &&
         FOLLY_IS_TRIVIALLY_COPYABLE(Value)) {