Add release note for WG14 N3298

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -274,6 +274,13 @@ C Language Changes
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^
 
+- Updated conformance for `N3298 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3298.htm>`_
+  which adds the ``i`` and ``j`` suffixes for the creation of a ``_Complex``
+  constant value. Clang has always supported these suffixes as a GNU extension,
+  so ``-Wgnu-imaginary-constant`` no longer has effect in C modes, as this is
+  not a C2y extension in C. ``-Wgnu-imaginary-constant`` still applies in C++
+  modes.
+
 - Clang updated conformance for `N3370 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3370.htm>`_
   case range expressions. This feature was previously supported by Clang as a
   GNU extension, so ``-Wgnu-case-range`` no longer has effect in C modes, as