The SubjectMatchRule enum should not be used as a DenseMap key to avoid

UBSAN 'invalid value' failures

The commit r300556 introduced a UBSAN issue that was caught by
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap. The DenseMap
failed to create an empty/tombstone value as the empty/tombstone values for the
SubjectMatchRule enum were not valid enum constants.

llvm-svn: 300591