[libc++][Android] Always redirect <stdatomic.h> to <atomic>

When targeting Android, enable the <stdatomic.h> to <atomic>
redirection for C++ language dialects before C++23, because this
redirection historically existed for Android's C++ toolchain.
Currently, the Android toolchain achieves this redirection with a
stdatomic.h in the sysroot (from Bionic) that defines the C API either
directly or by aliasing std identifiers from <atomic>. The Android
team's LLVM build scripts then copy Bionic's header over the one in the
Clang resource directory.

Hopefully, by moving Android's redirection into LLVM itself, the
Android stdatomic.h situation can be simplified eventually.

Author: rprichard

libcxx/include/atomic

@@ -598,7 +598,10 @@ template <class T>
 #    define _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED 0
 #  endif
 
-#  if _LIBCPP_STD_VER < 23 && _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED
+// The Android LLVM toolchain has historically allowed combining the <atomic>
+// and <stdatomic.h> headers in dialects before C++23, so for backwards
+// compatibility, preserve that ability when targeting Android.
+#  if _LIBCPP_STD_VER < 23 && !defined(__ANDROID__) && _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED
 #    error <atomic> is incompatible with <stdatomic.h> before C++23. Please compile with -std=c++23.
 #  endif
 

libcxx/include/stdatomic.h

@@ -126,7 +126,10 @@ using std::atomic_signal_fence                         // see below
 #    pragma GCC system_header
 #  endif
 
-#  if defined(__cplusplus) && _LIBCPP_STD_VER >= 23
+// The Android LLVM toolchain has historically allowed combining the <atomic>
+// and <stdatomic.h> headers in dialects before C++23, so for backwards
+// compatibility, preserve that ability when targeting Android.
+#  if defined(__cplusplus) && (_LIBCPP_STD_VER >= 23 || defined(__ANDROID__))
 
 #    include <atomic>
 #    include <version>

libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp

@@ -7,7 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: no-threads
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// UNSUPPORTED: c++03
+// UNSUPPORTED: (c++11 || c++14 || c++17 || c++20) && !android
 
 // This test verifies that <stdatomic.h> redirects to <atomic>.
 

libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp

@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20
 
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// XFAIL: android
+
 // No diagnostic gets emitted when we build with modules.
 // XFAIL: clang-modules-build
 

libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp

@@ -21,6 +21,9 @@
 // doesn't work at all if we don't use the <stdatomic.h> provided by libc++ in C++23 and above.
 // XFAIL: (c++11 || c++14 || c++17 || c++20) && gcc
 
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// XFAIL: android
+
 #include <atomic>
 #include <stdatomic.h>
 #include <type_traits>