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

Mark `force_iteration_on_noniterable_enum` as an `inline` variable
to avoid ODR violations.

Differential Revision: https://reviews.llvm.org/D131777

Author: JoeLoser

llvm/include/llvm/ADT/Sequence.h

@@ -104,8 +104,7 @@ 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 {