add missing constexpr to optional::value_or

[Credit to cpplearner]

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

llvm-svn: 304813
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: dd7c68bc7867adabc34bfe07c1a85c4cb955e088
diff --git a/include/optional b/include/optional
index 8c8ee76..c0fd0e7 100644
--- a/include/optional
+++ b/include/optional
@@ -897,7 +897,7 @@
 
     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");