Better OpenBSD frontend support

Summary:
- Like other *BSD, conditioning certain flags to pass
- To prepare future OpenBSD sanitisers.

Patch by: David CARLIER

Reviewers: dlj, krytarowski, vitalybuka

Reviewed By: vitalybuka

Subscribers: krytarowski, cfe-commits

Differential Revision: https://reviews.llvm.org/D43818

llvm-svn: 326645