[Driver] Handle default case in refactored addOpenMPRuntime

Summary:
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
early exit).

This fixes -Wswitch triggered by r371442.

Reviewers: srhines, danalbert, jdoerfert

Subscribers: guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67364

llvm-svn: 371444