[gcc r12-8463] libstdc++: Only include <ext/atomicity.h> for COW string

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:64f5d58d8151324be111ba6e13c50b4b642c41c2

commit r12-8463-g64f5d58d8151324be111ba6e13c50b4b642c41c2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu May 19 12:54:41 2022 +0100

    libstdc++: Only include <ext/atomicity.h> for COW string
    
    Since the COW std::string was moved to its own header, we don't need the
    atomic dispatch helpers in the definition of std::__cxx11::string. Move
    the inclusion of the <ext/atomicity.h> header to <bits/cow_string.h>
    where it's needed.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h: Do not include <ext/atomicity.h>
            here.
            * include/bits/cow_string.h: Include it here.
    
    (cherry picked from commit f3e22baec0290c23654e99bf184153765944f4aa)

Diff:
---
 libstdc++-v3/include/bits/basic_string.h | 1 -
 libstdc++-v3/include/bits/cow_string.h   | 2 ++
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index c3fbc53953c..6041d05815b 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -36,7 +36,6 @@
 
 #pragma GCC system_header
 
-#include <ext/atomicity.h>
 #include <ext/alloc_traits.h>
 #include <debug/debug.h>
 
diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index a49a5b04f2f..7bceb06a79b 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -34,6 +34,8 @@
 
 #if ! _GLIBCXX_USE_CXX11_ABI
 
+#include <ext/atomicity.h> // _Atomic_word, __is_single_threaded
+
 #ifdef __cpp_lib_is_constant_evaluated
 // Support P1032R1 in C++20 (but not P0980R1 for COW strings).
 # define __cpp_lib_constexpr_string 201811L