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>.

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