[libc++] Change vector<bool>::const_iterator::reference to bool in ABIv2

`vector<bool>::const_reference` and `vector<bool>::const_iterator::reference` should be the same type.

Reviewed By: Mordante, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D123851

NOKEYCHECK=True
GitOrigin-RevId: 907ed12d950f8e138620769420e5f84c0c7ece78
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index dc86c58..ddb785b 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -86,6 +86,9 @@
   supported anymore. Please migrate to using the new support for
   :ref:`assertions <assertions-mode>` instead.
 
+- ``vector<bool>::const_reference``, ``vector<bool>::const_iterator::reference``
+  and ``bitset::const_reference`` are now aliases for `bool` in the unstable ABI.
+
 ABI Changes
 -----------