Add temporary workaround for AppleClang 15 until we drop CI for it

Author: ldionne

libcxx/include/__algorithm/simd_utils.h

@@ -26,7 +26,9 @@ _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 // TODO: Find out how altivec changes things and allow vectorizations there too.
-#if _LIBCPP_STD_VER >= 14 && defined(_LIBCPP_COMPILER_CLANG_BASED) && !defined(__ALTIVEC__)
+// TODO: Simplify this condition once we stop building with AppleClang 15 in the CI.
+#if _LIBCPP_STD_VER >= 14 && defined(_LIBCPP_COMPILER_CLANG_BASED) && !defined(__ALTIVEC__) &&                         \
+    !(defined(_LIBCPP_APPLE_CLANG_VER) && _LIBCPP_APPLE_CLANG_VER < 1600)
 #  define _LIBCPP_HAS_ALGORITHM_VECTOR_UTILS 1
 #else
 #  define _LIBCPP_HAS_ALGORITHM_VECTOR_UTILS 0

libcxx/test/libcxx/algorithms/vectorization.compile.pass.cpp

@@ -17,6 +17,9 @@
 // We don't vectorize algorithms on AIX right now.
 // XFAIL: target={{.+}}-aix{{.*}}
 
+// We don't vectorize on AppleClang 15 since that apparently breaks std::mismatch
+// XFAIL: apple-clang-15
+
 // This test ensures that we enable the vectorization of algorithms on the expected
 // platforms.