[obj2yaml] Recognize input file format using llvm::object::ObjectFile
interface methods isCOFF().

The '-coff' command line option has been removed. It was not used in any
test cases.

The patch reviewed by Michael Spencer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208157 91177308-0d34-0410-b5e6-96231b3b80d8