[libc++] Ensure that we vectorize algorithms on all Clang-based compilers

Otherwise, we wouldn't vectorize on compilers like AppleClang when in
reality we know perfectly well how to do it.

Author: ldionne

libcxx/include/__algorithm/simd_utils.h

@@ -26,7 +26,7 @@ _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_CLANG_VER) && !defined(__ALTIVEC__)
+#if _LIBCPP_STD_VER >= 14 && defined(_LIBCPP_COMPILER_CLANG_BASED) && !defined(__ALTIVEC__)
 #  define _LIBCPP_HAS_ALGORITHM_VECTOR_UTILS 1
 #else
 #  define _LIBCPP_HAS_ALGORITHM_VECTOR_UTILS 0

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

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// We don't know how to vectorize algorithms on GCC
+// XFAIL: gcc
+
+// This test ensures that we enable the vectorization of algorithms on the expected
+// platforms.
+
+#include <algorithm>
+
+#if !_LIBCPP_VECTORIZE_ALGORITHMS
+#  error Algorithms should be vectorized on this platform
+#endif