[Driver] Handle default case in refactored addOpenMPRuntime

[pirama]

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.

[pirama]

Check for OMPRT_Unknown instead of a default case.

clang/lib/Driver/ToolChains/CommonArgs.cpp

case Driver::OMPRT_OMP:
     CmdArgs.push_back("-lomp");
     break;
   case Driver::OMPRT_GOMP:
     CmdArgs.push_back("-lgomp");
     break;
   case Driver::OMPRT_IOMP5:
     CmdArgs.push_back("-liomp5");
     break;
+  case Driver::OMPRT_Unknown:
+    break;
   }
 
   if (ForceStaticHostRuntime)
     CmdArgs.push_back("-Bdynamic");
 
   if (RTKind == Driver::OMPRT_GOMP && GompNeedsRT)
       CmdArgs.push_back("-lrt");