Update release notes

Author: redcode

clang/docs/ReleaseNotes.rst

@@ -471,6 +471,17 @@ Non-comprehensive list of changes in this release
   ``__builtin_elementwise_sub_sat``, ``__builtin_reduce_min`` (For integral element type),
   ``__builtin_reduce_max`` (For integral element type).
 
+- The builtin macros ``__INT8_C``, ``__INT16_C``, ``__INT32_C``, ``__INT64_C``,
+  ``__INTMAX_C``, ``__UINT8_C``, ``__UINT16_C``, ``__UINT32_C``, ``__UINT64_C``
+  and ``__UINTMAX_C`` have been introduced to ease the implementaton of section
+  7.18.4 of ISO/IEC 9899:1999. These macros are also defined by GCC and should
+  be used instead of others that need to expand and paste the suffixes provided
+  by ``__INT8_C_SUFFIX__``, ``__INT16_C_SUFFIX__``, ``__INT32_C_SUFFIX__``,
+  ``__INT64_C_SUFFIX__``, ``__INTMAX_C_SUFFIX__``, ``__UINT8_C_SUFFIX__``,
+  ``__UINT16_C_SUFFIX__``, ``__UINT32_C_SUFFIX__``, ``__UINT64_C_SUFFIX__`` and
+  ``__UINTMAX_C_SUFFIX__``. Pasting suffixes is unsafe, as users can define them
+  as macros.
+
 - Clang now rejects ``_BitInt`` matrix element types if the bit width is less than ``CHAR_WIDTH`` or
   not a power of two, matching preexisting behaviour for vector types.