[libc++] Fix test FTM header guard

That template is actually not used to generate the version header yet,
but we can at least fix the include guards which are clearly incorrect.

libcxx/test/libcxx/feature_test_macro/version_header.sh.py

@@ -30,8 +30,8 @@ test(
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef _LIBCPP_VERSION
-#define _LIBCPP_VERSION
+#ifndef _LIBCPP_VERSIONH
+#define _LIBCPP_VERSIONH
 
 #include <__config>
 
@@ -69,6 +69,6 @@ test(
 // define __cpp_lib_missing_FTM_in_older_standard 2026L
 #endif // _LIBCPP_STD_VER >= 26
 
-#endif // _LIBCPP_VERSION
+#endif // _LIBCPP_VERSIONH
 """,
 )

libcxx/utils/generate_feature_test_macro_components.py

@@ -2230,8 +2230,8 @@ class FeatureTestMacros:
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef _LIBCPP_VERSION
-#define _LIBCPP_VERSION
+#ifndef _LIBCPP_VERSIONH
+#define _LIBCPP_VERSIONH
 
 #include <__config>
 
@@ -2241,7 +2241,7 @@ class FeatureTestMacros:
 
 {feature_test_macros}
 
-#endif // _LIBCPP_VERSION
+#endif // _LIBCPP_VERSIONH
 """
         return template.format(
             feature_test_macros=generate_version_header_implementation(