Fix build bot after r319750 "[Support/TarWriter] - Don't allow TarWriter to...

Fix build bot after r319750 "[Support/TarWriter] - Don't allow TarWriter to add the same file more than once."

Error was:
error: comparison of integers of different signs: 'const unsigned long' and 'const int' [-Werror,-Wsign-compare]

http://lab.llvm.org:8011/builders/ubuntu-gcc7.1-werror/builds/3469/steps/build-unified-tree/logs/stdio
http://lab.llvm.org:8011/builders/clang-with-thin-lto-ubuntu/builds/7118/steps/build-stage2-compiler/logs/stdio

llvm-svn: 319752