[libcxx] Fix allocator<void>::pointer in C++20 with removed members
When compiled with `-D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`
uses of `allocator<void>::pointer` resulted in compiler errors after D104323.
If we instantiate the primary template, `allocator<void>::reference` produces
an error 'cannot form references to void'.
To workaround this, allow to bring back the `allocator<void>` specialization by defining the new `_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION` macro.
To make sure the code that uses `allocator<void>` and the removed members does not break,
both `_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS` and `_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS` have to be defined.
Reviewed By: ldionne, #libc, philnik
Differential Revision: https://reviews.llvm.org/D126210
NOKEYCHECK=True
GitOrigin-RevId: 374f938fe8f2beefbac8de9faf5d79fea8469e3d
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 362e0ce..bae9567 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -136,6 +136,11 @@
against the static version of libc++, which will result in no dependency being
taken against the shared library.
+- The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION`` macro has been added to allow
+ re-enabling the ``allocator<void>`` specialization. When used in conjuction with
+ ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS``, this ensures that the members of
+ ``allocator<void>`` removed in C++20 can be accessed.
+
ABI Changes
-----------