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

(cherry picked from commit 4bb3a1e)

Author: chrisnc

clang/docs/ReleaseNotes.rst

@@ -891,6 +891,9 @@ Bug Fixes in This Version
 - Fixed an assertion failure when a template non-type parameter contains
   an invalid expression.
 
+- 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

@@ -172,7 +172,11 @@ 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

clang/test/Headers/stdatomic.c

@@ -1,5 +1,8 @@
 // RUN: %clang_cc1 -std=c11 -E %s | FileCheck %s
 // RUN: %clang_cc1 -std=c11 -fms-compatibility -E %s | FileCheck %s
+// RUN: %clang_cc1 -std=c11 %s -verify
+// RUN: %clang_cc1 -x c++ -std=c++11 %s -verify
+// expected-no-diagnostics
 #include <stdatomic.h>
 
 int bool_lock_free = ATOMIC_BOOL_LOCK_FREE;
@@ -31,3 +34,5 @@ int llong_lock_free = ATOMIC_LLONG_LOCK_FREE;
 
 int pointer_lock_free = ATOMIC_POINTER_LOCK_FREE;
 // CHECK: pointer_lock_free = {{ *[012] *;}}
+
+atomic_flag f = ATOMIC_FLAG_INIT;