[libc++] Build and test with -Wundef warning. NFC.

This will avoid typos like `_LIBCPP_STD_VERS` (<future>) or using `#if TEST_STD_VER > 17` without including "test_macros.h".

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D99515

GitOrigin-RevId: 5c703f0fd81960386cc02b9ca618e866c9d5e29a
diff --git a/include/experimental/simd b/include/experimental/simd
index 41f8f79..d1aaf55 100644
--- a/include/experimental/simd
+++ b/include/experimental/simd
@@ -725,12 +725,12 @@
 
 template <class _Tp, size_t __bytes>
 struct __vec_ext_traits {
-#if !defined(_LIBCPP_COMPILER_CLANG)
+#if !defined(_LIBCPP_COMPILER_CLANG_BASED)
   typedef _Tp type __attribute__((vector_size(__ceil_pow_of_2(__bytes))));
 #endif
 };
 
-#if defined(_LIBCPP_COMPILER_CLANG)
+#if defined(_LIBCPP_COMPILER_CLANG_BASED)
 #define _LIBCPP_SPECIALIZE_VEC_EXT(_TYPE, _NUM_ELEMENT)                        \
   template <>                                                                  \
   struct __vec_ext_traits<_TYPE, sizeof(_TYPE) * _NUM_ELEMENT> {               \