[clang] Add release note for int->enum conversion change. (#144407)

This seems to be having some practical impact, so we should let people
know.

Author: efriedma-quic

clang/docs/ReleaseNotes.rst

@@ -65,8 +65,10 @@ C++ Specific Potentially Breaking Changes
   standard library already have their own bespoke builtins.
 - A workaround for libstdc++4.7 has been removed. Note that 4.8.3 remains the oldest
   supported libstdc++ version.
-
 - Added ``!nonnull/!align`` metadata to load of references for better codegen.
+- Checking for int->enum conversions in constant expressions is more strict;
+  in particular, ``const E x = (E)-1;`` is not treated as a constant if it's
+  out of range. This impacts old versions of Boost.  (#GH143034)
 
 ABI Changes in This Version
 ---------------------------