Bug 1319016 - Part1 - Make IntegralConstant::value use constexpr. r=froydnj

MozReview-Commit-ID: DtrDeMC1f5T

--HG--
extra : rebase_source : 820fb6a1a3562bb3d6aa62294338f1328f0fb1c4

mfbt/TypeTraits.h

@@ -46,7 +46,7 @@ typename AddRvalueReference<T>::Type DeclVal();
 template<typename T, T Value>
 struct IntegralConstant
 {
-  static const T value = Value;
+  static constexpr T value = Value;
   typedef T ValueType;
   typedef IntegralConstant<T, Value> Type;
 };