fix a couple of -Wunused-value violations

Summary:
Reported by clang-12:
```
folly/test/SynchronizedTestLib-inl.h:587:7: error: expression result unused [-Werror,-Wunused-value]
      SYNCHRONIZED_DUAL(lv, v, lm, m) {
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
folly/Synchronized.h:1819:5: note: expanded from macro 'SYNCHRONIZED_DUAL'
    ::folly::detail::SYNCHRONIZED_macro_is_deprecated{};                       \
    ^                                                ~~
```

Reviewed By: chadaustin

Differential Revision: D27954777

fbshipit-source-id: bd1dba5ebf563b34b496457b980568ea1c978442

Author: Igor Sugak

folly/Synchronized.h

@@ -1781,7 +1781,7 @@ struct [[deprecated(
   FOLLY_MSVC_DISABLE_WARNING(4459) /* declaration hides global */         \
   FOLLY_GCC_DISABLE_NEW_SHADOW_WARNINGS                                   \
   if (bool SYNCHRONIZED_VAR(state) = false) {                             \
-    ::folly::detail::SYNCHRONIZED_macro_is_deprecated{};                  \
+    (void)::folly::detail::SYNCHRONIZED_macro_is_deprecated{};            \
   } else                                                                  \
     for (auto SYNCHRONIZED_VAR(lockedPtr) =                               \
              (FB_VA_GLUE(FB_ARG_2_OR_1, (__VA_ARGS__))).contextualLock(); \
@@ -1816,7 +1816,7 @@ struct [[deprecated(
  */
 #define SYNCHRONIZED_DUAL(n1, e1, n2, e2)                                      \
   if (bool SYNCHRONIZED_VAR(state) = false) {                                  \
-    ::folly::detail::SYNCHRONIZED_macro_is_deprecated{};                       \
+    (void)::folly::detail::SYNCHRONIZED_macro_is_deprecated{};                 \
   } else                                                                       \
     for (auto SYNCHRONIZED_VAR(ptrs) = acquireLockedPair(e1, e2);              \
          !SYNCHRONIZED_VAR(state);                                             \