swift-foundation's include of `stdatomic` breaks on windows when C++ interoperability is enabled

[hyp]

Description

Trying to import new foundation on Windows, with the fix for #75691, I hit a follow-up issue related to stdatomic that complains about conflicting types for 'atomic_thread_fence':

# command stderr:
<unknown>:0: warning: the '-enable-experimental-cxx-interop' flag is deprecated; please pass '-cxx-interoperability-mode=' instead
<unknown>:0: note: Swift will maintain source compatibility for imported APIs based on the selected compatibility mode, so updating the Swift compiler will not change how APIs are imported
path arg : 'S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk'
<module-includes>:1:10: note: in file included from <module-includes>:1:
1 | #include "_FoundationCShims.h"
  |          `- note: in file included from <module-includes>:1:
2 |

S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_FoundationCShims.h:17:10: note: in file included from S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_FoundationCShims.h:17:
15 |
16 | #include "_CShimsTargetConditionals.h"
17 | #include "_CStdlib.h"
   |          `- note: in file included from S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_FoundationCShims.h:17:
18 | #include "CFUniCharBitmapData.inc.h"
19 | #include "CFUniCharBitmapData.h"

S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_CStdlib.h:127:10: note: in file included from S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_CStdlib.h:127:
125 | //#if !defined(_WIN32)
126 | #if __has_include(<stdatomic.h>)
127 | #include <stdatomic.h>
    |          `- note: in file included from S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_CStdlib.h:127:
128 | #endif
129 | //#endif

s:\b\5\lib\swift\clang\include\stdatomic.h:81:6: error: conflicting types for 'atomic_thread_fence'
 79 |
 80 | /* These should be provided by the libc implementation. */
 81 | void atomic_thread_fence(memory_order);
    |      `- error: conflicting types for 'atomic_thread_fence'
 82 | void atomic_signal_fence(memory_order);
 83 |

C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.40.33807\include/atomic:337:36: note: previous definition is here
 335 | #endif // _CMPXCHG_MASK_OUT_PADDING_BITS


 336 |


 337 | _EXPORT_STD extern "C" inline void atomic_thread_fence(const memory_order _Order) noexcept {


     |                                    `- note: previous definition is here
 338 |     ::_Atomic_thread_fence(static_cast<unsigned int>(_Order));


 339 | }



<module-includes>:1:10: note: in file included from <module-includes>:1:
1 | #include "_FoundationCShims.h"
  |          `- note: in file included from <module-includes>:1:
2 |

S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_FoundationCShims.h:17:10: note: in file included from S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_FoundationCShims.h:17:
15 |
16 | #include "_CShimsTargetConditionals.h"
17 | #include "_CStdlib.h"
   |          `- note: in file included from S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_FoundationCShims.h:17:
18 | #include "CFUniCharBitmapData.inc.h"
19 | #include "CFUniCharBitmapData.h"

S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_CStdlib.h:127:10: note: in file included from S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_CStdlib.h:127:
125 | //#if !defined(_WIN32)
126 | #if __has_include(<stdatomic.h>)
127 | #include <stdatomic.h>
    |          `- note: in file included from S:\Program Files\Swift\Platforms\Windows.platform\Developer\SDKs\Windows.sdk\usr\include\_FoundationCShims/_CStdlib.h:127:
128 | #endif
129 | //#endif

<unknown>:0: error: could not build C module '_FoundationCShims'
s:\b\5\lib\swift\clang\include\stdatomic.h:82:6: error: conflicting types for 'atomic_signal_fence'
 80 | /* These should be provided by the libc implementation. */
 81 | void atomic_thread_fence(memory_order);
 82 | void atomic_signal_fence(memory_order);
    |      `- error: conflicting types for 'atomic_signal_fence'
 83 |
 84 | #define atomic_thread_fence(order) __c11_atomic_thread_fence(order)

C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.40.33807\include/atomic:341:36: note: previous definition is here
 339 | }


 340 |


 341 | _EXPORT_STD extern "C" inline void atomic_signal_fence(const memory_order _Order) noexcept {


     |                                    `- note: previous definition is here
 342 |     if (_Order != memory_order_relaxed) {


 343 |         _Compiler_barrier();



error: command failed with exit status: 1

Reproduction

import Foundation and enable C++ interop, windows host.

Expected behavior

It should work.

Environment

Swift main.

Additional information

No response

[hyp]

I think the right fix is to make the two definitions between clang and MSVC match, but for now let's workaround this in the new foundation by not including stdatomic