[Driver] Handle default case in refactored addOpenMPRuntime
Appease failed builds (due to -Werror and -Wswitch) where OMPRT_Unknown
is not handled in the switch statement (even though it's handled by the
This fixes -Wswitch triggered by r371442.
Reviewers: srhines, danalbert, jdoerfert
Subscribers: guansong, cfe-commits
Differential Revision: https://reviews.llvm.org/D67364