[gcc r12-7064] libstdc++: Allow Clang to use <stdatomic.h> before C++23

[Jonathan Wakely]

https://gcc.gnu.org/g:0d1aabb60ec66bffbc9a3269f6ca85150f259564

commit r12-7064-g0d1aabb60ec66bffbc9a3269f6ca85150f259564
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Feb 4 19:42:19 2022 +0000

    libstdc++: Allow Clang to use <stdatomic.h> before C++23
    
    There is code that only expects to be compiled with clang++ and uses its
    <stdatomic.h>, which works because Clang supports the _Atomic specifier
    in C++. The addition  of <stdatomic.h> to libstdc++ broke this code, as
    now it finds the C++ header instead, which is empty for any standard
    mode before C++23.
    
    This change allows that code to keep working as before, by forwarding to
    clang's <stdatomic.h>.
    
    libstdc++-v3/ChangeLog:
    
            * include/c_compatibility/stdatomic.h [__clang__]: Use
            #include_next <stdatomic.h>.

Diff:
---
 libstdc++-v3/include/c_compatibility/stdatomic.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/include/c_compatibility/stdatomic.h b/libstdc++-v3/include/c_compatibility/stdatomic.h
index 852574bd87c..95c72615b4e 100644
--- a/libstdc++-v3/include/c_compatibility/stdatomic.h
+++ b/libstdc++-v3/include/c_compatibility/stdatomic.h
@@ -120,5 +120,7 @@ using std::atomic_flag_clear_explicit;
 using std::atomic_thread_fence;
 using std::atomic_signal_fence;
 
+#elif defined __clang__
+# include_next <stdatomic.h>
 #endif // C++23
 #endif // _GLIBCXX_STDATOMIC_H