[clang] Define ATOMIC_FLAG_INIT correctly for C++. (llvm#97534)

(cherry picked from commit 4bb3a1e)

Author: chrisnc

clang/docs/ReleaseNotes.rst

@@ -139,6 +139,9 @@ Improvements to Coverage Mapping
 Bug Fixes in This Version
 -------------------------
 
+- Fixed the definition of ``ATOMIC_FLAG_INIT`` in ``<stdatomic.h>`` so it can
+  be used in C++.
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^