folly/Synchronized.h: avoid shadowing warnings

Summary:
Nested uses of SYNCHRONIZED (and related) macros leads
inevitably to shadowed variable declarations.
There is already a use of FOLLY_GCC_DISABLE_WARNING(shadow)
to disable -Wshadow in that code, but obviously, that cannot
help with the new options, so do similar for them via
the new FOLLY_GCC_DISABLE_NEW_SHADOW_WARNINGS.

Depends on D4041696

Reviewed By: markisaa

Differential Revision: D4041728

fbshipit-source-id: b85fb3452f6855d359f9b910abbd02cf8433f3f7

folly/Synchronized.h

@@ -1302,6 +1302,7 @@ void swap(Synchronized<T, M>& lhs, Synchronized<T, M>& rhs) {
 #define SYNCHRONIZED(...)                                             \
   FOLLY_PUSH_WARNING                                                  \
   FOLLY_GCC_DISABLE_WARNING(shadow)                                   \
+  FOLLY_GCC_DISABLE_NEW_SHADOW_WARNINGS                               \
   if (bool SYNCHRONIZED_state = false) {                              \
   } else                                                              \
     for (auto SYNCHRONIZED_lockedPtr =                                \