NFC: Use range-based for loop in test

Author: Hubert Tong

libcxxabi/test/test_demangle.pass.cpp

@@ -30407,13 +30407,15 @@ void test_xfail_cases()
 {
     std::size_t len = 0;
     char* buf = nullptr;
-    for (const char* const* it = xfail_cases; *it; ++it)
+    for (auto c_str : xfail_cases)
     {
+        if (!c_str)
+            break;
         int status;
-        char* demang = __cxxabiv1::__cxa_demangle(*it, buf, &len, &status);
+        char* demang = __cxxabiv1::__cxa_demangle(c_str, buf, &len, &status);
         if (status != -2)
         {
-            std::printf("%s was documented as xfail but passed\n", *it);
+            std::printf("%s was documented as xfail but passed\n", c_str);
             std::printf("Got status = %d\n", status);
             assert(status == -2);
         }