Fix build bot problem with missing OMPC_novariants in switch.

Author: jyu2-git

clang/lib/CodeGen/CGStmtOpenMP.cpp

@@ -5612,6 +5612,7 @@ static void emitOMPAtomicExpr(CodeGenFunction &CGF, OpenMPClauseKind Kind,
   case OMPC_notinbranch:
   case OMPC_link:
   case OMPC_use:
+  case OMPC_novariants:
     llvm_unreachable("Clause is not allowed in 'omp atomic'.");
   }
 }