Use a typed enum instead of 'unsigned char' for packed field.  NFC.

This makes it easier to debug Twine as the 'Kind' fields now show their enum values in lldb and not escaped characters.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223178 91177308-0d34-0410-b5e6-96231b3b80d8