work around to support debian's fork of llvm 7.0.1

it has a patch that adds an OS type, breaking the public API
this commit avoids depending on the last os type enum item,
but retains the safety assertion checks.

closes #1788
c54fe0d3aeAndrew Kelley committed on 11/29/2018, 3:58:52 AMparent11e8afb
1 file changedLine totals unavailable