Assert that MicroSpinLock is a POD type

Summary: Because if it's not, all hell will break lose (apparently).

Reviewed By: meyering

Differential Revision: D3373061

fbshipit-source-id: f08a863d1bacadeeff9d9cea041f399f136cfc74

folly/MicroSpinLock.h

@@ -105,6 +105,9 @@ struct MicroSpinLock {
                                                         std::memory_order_relaxed);
   }
 };
+static_assert(
+    std::is_pod<MicroSpinLock>::value,
+    "MicroSpinLock must be kept a POD type.");
 
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