work around to support debian's fork of llvm 7.0.1
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

raulgrell/zig,zig-lang/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig