Add release note

Author: tbaederr

clang/docs/ReleaseNotes.rst

@@ -148,6 +148,17 @@ C++ Specific Potentially Breaking Changes
     // Now diagnoses with an error.
     void f(int& i [[clang::lifetimebound]]);
 
+- Clang now rejects all field accesses on null pointers in constant expressions. The following code
+  used to work but will now be rejected:
+
+  .. code-block:: c++
+
+    struct S { int a; int b; };
+    constexpr const int *p = &((S*)nullptr)->b;
+
+  Previously, this code was erroneously accepted.
+
+
 ABI Changes in This Version
 ---------------------------