Add llvm_unreachable at end of fully covered switch

To pacify GCC warning about control reaching end of non-void function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181197 91177308-0d34-0410-b5e6-96231b3b80d8
Remove forward slashes from check; should unbreak Windows buildbots.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181199 91177308-0d34-0410-b5e6-96231b3b80d8