When encountering an unknown file format, ObjectFile::createObjectFile should
politely report it instead of running into llvm_unreachable.

Also patch llvm-dwarfdump to actually check whether the file it's attempting to
dump is a valid object file.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173489 91177308-0d34-0410-b5e6-96231b3b80d8