[clang] Define ATOMIC_FLAG_INIT correctly for C++.

Author: chrisnc

clang/docs/ReleaseNotes.rst

@@ -750,6 +750,8 @@ Bug Fixes in This Version
 
 - Fixed `static_cast` to array of unknown bound. Fixes (#GH62863).
 
+- Fixed the definition of ATOMIC_FLAG_INIT in stdatomic.h so it can be used in C++.
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Headers/stdatomic.h

@@ -166,7 +166,11 @@ typedef _Atomic(uintmax_t)          atomic_uintmax_t;
 
 typedef struct atomic_flag { atomic_bool _Value; } atomic_flag;
 
+#ifdef __cplusplus
+#define ATOMIC_FLAG_INIT {false}
+#else
 #define ATOMIC_FLAG_INIT { 0 }
+#endif
 
 /* These should be provided by the libc implementation. */
 #ifdef __cplusplus