[libc++] Remove overly conservative error in <stdatomic.h>

As mentionned in D97044, it is fine if users include <atomic> and then
include <stdatomic.h> -- we don't need to error out for that case.

Differential Revision: https://reviews.llvm.org/D125579

libcxx/include/stdatomic.h

@@ -226,14 +226,7 @@ using std::atomic_thread_fence _LIBCPP_USING_IF_EXISTS;
 // the header. We do this because Clang has historically shipped a <stdatomic.h>
 // header that would be available in all Standard modes, and we don't want to
 // break that use case.
-//
-// However, if the user has already used <atomic> before, the two headers are
-// incompatible before C++23, so we issue a clear error here to avoid obscure
-// issues down the line.
 # if __has_include_next(<stdatomic.h>)
-#   ifdef _LIBCPP_ATOMIC
-#     error <stdatomic.h> is incompatible with <atomic> before C++23. Please compile with -std=c++23.
-#   endif
 #   include_next <stdatomic.h>
 # endif
 

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

@@ -9,9 +9,9 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20
 
-// This test ensures that we issue a reasonable diagnostic when using <atomic> while <stdatomic.h>
-// is in use too. Before C++23, this otherwise leads to obscure errors because <atomic> may try
-// to redefine things defined by <stdatomic.h>.
+// This test ensures that we issue a reasonable diagnostic when including <atomic> after
+// <stdatomic.h> has been included. Before C++23, this otherwise leads to obscure errors
+// because <atomic> may try to redefine things defined by <stdatomic.h>.
 
 // Ignore additional weird errors that happen when the two headers are mixed.
 // ADDITIONAL_COMPILE_FLAGS: -Xclang -verify-ignore-unexpected=error -Xclang -verify-ignore-unexpected=warning