silence -Wreturn-type warnings

These codepaths would generate warnings with GCC on linux even though the switch
was covered.  Add llvm_unreachable markers to indicate that the switch should be
covered.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260865 91177308-0d34-0410-b5e6-96231b3b80d8