Add constexpr to pair

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D80558

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 737a4501e815d8dd57e5095dbbbede500dfa8ccb
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index a6867fb..1dbeb21 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -184,6 +184,8 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_swap_algorithms``           *unimplemented*  
     ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_utility ``                  ``201811L``      
+    ------------------------------------------------- -----------------
     ``__cpp_lib_destroying_delete``                   ``201806L``      
     ------------------------------------------------- -----------------
     ``__cpp_lib_endian``                              ``201907L``