Address review comment.

Author: mordante

libcxx/include/__cxx03/__config

@@ -1218,7 +1218,8 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 #  endif
 
-#  if defined(__cpp_explicit_this_parameter)
+// Clang-18 has support for deducing this, but it does not set the FTM.
+#  if defined(__cpp_explicit_this_parameter) || (defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER >= 1800)
 #    define _LIBCPP_HAS_EXPLICIT_THIS_PARAMETER
 #  endif