[libc++][stdatomic.h] Implements LWG3671. (#99775)

Implements:
- LWG3671 atomic_fetch_xor missing from stdatomic.h

Author: mordante

libcxx/docs/Status/Cxx23Issues.csv

@@ -164,7 +164,7 @@
 "`3656 <https://wg21.link/LWG3656>`__","Inconsistent bit operations returning a count","July 2022","|Complete|","15.0",""
 "`3659 <https://wg21.link/LWG3659>`__","Consider ``ATOMIC_FLAG_INIT`` undeprecation","July 2022","|Complete|","15.0"
 "`3670 <https://wg21.link/LWG3670>`__","``Cpp17InputIterators`` don't have integer-class difference types","July 2022","","","|ranges|"
-"`3671 <https://wg21.link/LWG3671>`__","``atomic_fetch_xor`` missing from ``stdatomic.h``","July 2022","",""
+"`3671 <https://wg21.link/LWG3671>`__","``atomic_fetch_xor`` missing from ``stdatomic.h``","July 2022","|Complete|","20.0"
 "`3672 <https://wg21.link/LWG3672>`__","``common_iterator::operator->()`` should return by value","July 2022","|Complete|","19.0","|ranges|"
 "`3683 <https://wg21.link/LWG3683>`__","``operator==`` for ``polymorphic_allocator`` cannot deduce template argument in common cases","July 2022","|Complete|","20.0"
 "`3687 <https://wg21.link/LWG3687>`__","``expected<cv void, E>`` move constructor should move","July 2022","|Complete|","16.0"

libcxx/include/stdatomic.h

@@ -103,6 +103,8 @@ using std::atomic_fetch_sub                            // see below
 using std::atomic_fetch_sub_explicit                   // see below
 using std::atomic_fetch_or                             // see below
 using std::atomic_fetch_or_explicit                    // see below
+using std::atomic_fetch_xor                            // see below
+using std::atomic_fetch_xor_explicit                   // see below
 using std::atomic_fetch_and                            // see below
 using std::atomic_fetch_and_explicit                   // see below
 using std::atomic_flag_test_and_set                    // see below
@@ -204,6 +206,8 @@ using std::atomic_fetch_add_explicit _LIBCPP_USING_IF_EXISTS;
 using std::atomic_fetch_and _LIBCPP_USING_IF_EXISTS;
 using std::atomic_fetch_and_explicit _LIBCPP_USING_IF_EXISTS;
 using std::atomic_fetch_or _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_xor_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_xor _LIBCPP_USING_IF_EXISTS;
 using std::atomic_fetch_or_explicit _LIBCPP_USING_IF_EXISTS;
 using std::atomic_fetch_sub _LIBCPP_USING_IF_EXISTS;
 using std::atomic_fetch_sub_explicit _LIBCPP_USING_IF_EXISTS;

libcxx/test/std/atomics/stdatomic.h.syn/types.compile.pass.cpp

@@ -101,6 +101,8 @@
 // using std::atomic_fetch_sub_explicit                   // see below
 // using std::atomic_fetch_or                             // see below
 // using std::atomic_fetch_or_explicit                    // see below
+// using std::atomic_fetch_xor                            // see below
+// using std::atomic_fetch_xor_explicit                   // see below
 // using std::atomic_fetch_and                            // see below
 // using std::atomic_fetch_and_explicit                   // see below
 // using std::atomic_flag_test_and_set                    // see below
@@ -222,6 +224,8 @@ void f() {
   using ::atomic_fetch_or_explicit;
   using ::atomic_fetch_sub;
   using ::atomic_fetch_sub_explicit;
+  using ::atomic_fetch_xor;
+  using ::atomic_fetch_xor_explicit;
   using ::atomic_flag_clear;
   using ::atomic_flag_clear_explicit;
   using ::atomic_flag_test_and_set;