[libc++] Implement P0627R6 (Function to mark unreachable code)

Reviewed By: ldionne, Quuxplusone, #libc

Spies: arichardson, mstorsjo, libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D119152

NOKEYCHECK=True
GitOrigin-RevId: 2a8f9a5e95dec3adcdee603358dc03def6d2245e
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 58b4506..0b77702 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -38,6 +38,8 @@
 New Features
 ------------
 
+ - Implemented P0627R6 (Function to mark unreachable code)
+
 API Changes
 -----------