Bug 1247338 - really make Atomic's constructor constexpr; r=Waldo

Atomic's constructor is marked as constexpr, but it calls a
non-constexpr function, ToStorageTypeArgument::convert.  For compilers
which require constexpr-ness on constructors to inline away the actual
constructor call, the call to ToStorageTypeArgument::convert completely
disables the constexpr-ness of the constructor.  Let's fix this by
marking all relevant instances of ToStorageTypeArgument::convert as
MOZ_CONSTEXPR, thus satisfying the compiler once again.

mfbt/Atomics.h

@@ -323,7 +323,7 @@ struct AtomicIntrinsics<T*, Order>
 template<typename T>
 struct ToStorageTypeArgument
 {
-  static T convert (T aT) { return aT; }
+  static MOZ_CONSTEXPR T convert (T aT) { return aT; }
 };
 
 } // namespace detail
@@ -514,13 +514,13 @@ struct ToStorageTypeArgument
 {
   typedef typename AtomicStorageType<T>::Type ResultType;
 
-  static ResultType convert (T aT) { return ResultType(aT); }
+  static MOZ_CONSTEXPR ResultType convert (T aT) { return ResultType(aT); }
 };
 
 template<typename T>
 struct ToStorageTypeArgument<T, false>
 {
-  static T convert (T aT) { return aT; }
+  static MOZ_CONSTEXPR T convert (T aT) { return aT; }
 };
 
 } // namespace detail