Clang: Add release note for defaulted-special-members-POD GCC ABI fix

Follow-up to 7846d59

Author: dwblaikie

clang/docs/ReleaseNotes.rst

@@ -609,6 +609,10 @@ ABI Changes in Clang
  such packing. Clang now matches the gcc behavior (except on Darwin and PS4).
  You can switch back to the old ABI behavior with the flag:
  ``-fclang-abi-compat=15.0``.
+- GCC allows POD types to have defaulted special members. Clang historically
+ classified such types as non-POD. Clang now matches the gcc behavior (except
+ on Darwin and PS4). You can switch back to the old ABI behavior with the flag:
+ ``-fclang-abi-compat=15.0``.
 
 OpenMP Support in Clang
 -----------------------