[ADT] Mark variable inline to avoid ODR violations in Sequence.h

[jloser]

Summary

Mark force_iteration_on_noniterable_enum as an inline variable
to avoid ODR violations.

llvm/include/llvm/ADT/Sequence.h

 template <typename EnumT> struct enum_iteration_traits {
   static constexpr bool is_iterable = false;
 };
 
 struct force_iteration_on_noniterable_enum_t {
   explicit force_iteration_on_noniterable_enum_t() = default;
 };
 
-// TODO: Make this `inline` once we update to C++17 to avoid ORD violations.
-constexpr force_iteration_on_noniterable_enum_t
+inline constexpr force_iteration_on_noniterable_enum_t
     force_iteration_on_noniterable_enum;
 
 namespace detail {
 
 // Returns whether a value of type U can be represented with type T.
 template <typename T, typename U> bool canTypeFitValue(const U Value) {
   const intmax_t BotT = intmax_t(std::numeric_limits<T>::min());
   const intmax_t BotU = intmax_t(std::numeric_limits<U>::min());