Fix return after llvm_unreachable

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192414 91177308-0d34-0410-b5e6-96231b3b80d8
Use switch instead of if, and handle all enum values.

This patch also handles errors other than no_more_files error. They were silently
ignored.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192415 91177308-0d34-0410-b5e6-96231b3b80d8
Remove an unused variable added in r192415.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192416 91177308-0d34-0410-b5e6-96231b3b80d8