[Debugify] Move interface definitions to a header, NFC

This is a minor cleanup in preparation for a change to export DI
statistics from -check-debugify. To do that, it would be cleaner to have
a dedicated header for the debugify interface.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337786 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed