[libc++] Consolidate the different [[nodiscard]] configuration options into a single one

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D129054

NOKEYCHECK=True
GitOrigin-RevId: b978dfbf749fdaa55c8f65793f6c7608c95ccb5b
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index f69b4d9..a89d4cd 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -55,6 +55,10 @@
   functions instead of as hidden friends, in conformance with the C++ standard.
   Also see `issue 56187 <https://github.com/llvm/llvm-project/issues/56187>`_.
 
+- ``_LIBCPP_ENABLE_NODISCARD`` and ``_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17`` are no longer respected.
+  Any standards-required ``[[nodiscard]]`` applications in C++20 are now always enabled. Any extended applications
+  can now be enabled by defining ``_LIBCPP_ENABLE_NODISCARD_EXT``.
+
 ABI Affecting Changes
 ---------------------