add missing constexpr to optional::value_or

[Credit to cpplearner]

Differential Revision: https://reviews.llvm.org/D27850

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@304813 91177308-0d34-0410-b5e6-96231b3b80d8

include/optional

@@ -897,7 +897,7 @@ public:
 
     template <class _Up>
     _LIBCPP_INLINE_VISIBILITY
-    value_type value_or(_Up&& __v) &&
+    constexpr value_type value_or(_Up&& __v) &&
     {
         static_assert(is_move_constructible_v<value_type>,
                       "optional<T>::value_or: T must be move constructible");