Removing default case statement from covered switch.

Previous patch did not count on the llvm command line parser to restrict the
inputs, but it is safe to do so.

Fix forward for patch with details:
-- https://reviews.llvm.org/D38650 and
-- https://llvm.org/svn/llvm-project/llvm/trunk@315635 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-svn: 315644