[NFC] Initialize a variable to make buildbot green.

In r268085 "[MS] Make #pragma pack use PragmaStack<> class." there was an
uninitialized variable 'Alignment', which caused the following failure:
  http://lab.llvm.org:8011/builders/sanitizer-ppc64be-linux/builds/1758
Zero-initialize the variable to fix this failure.

llvm-svn: 268129