[tools] Ignore -Wcast-qual in c-index-test.c after D153911 (NFC)

/Users/jiefu/llvm-project/clang/tools/c-index-test/c-index-test.c:234:18: error: cast from 'const char *' to 'char *' drops const qualifier [-Werror,-Wcast-qual]
    free((char *)unsaved_files[i].Filename);
                 ^
/Users/jiefu/llvm-project/clang/tools/c-index-test/c-index-test.c:235:18: error: cast from 'const char *' to 'char *' drops const qualifier [-Werror,-Wcast-qual]
    free((char *)unsaved_files[i].Contents);
                 ^
/Users/jiefu/llvm-project/clang/tools/c-index-test/c-index-test.c:3762:32: error: cast from 'const char *' to 'void *' drops const qualifier [-Werror,-Wcast-qual]
  return (CXIdxClientContainer)"TU";
                               ^
3 errors generated.