Change the llvm-obdump(1) behavior with the -macho flag and inappropriate file types.

To better match the old darwin otool(1) behavior, when llvm-obdump(1) is used
with the -macho option and the input file is not an object file simply print
the file name and this message:

foo: is not an object file

and continue on to process other input files.  Also in this case don’t exit
non-zero.  This should help in some OSS projects' with autoconf scripts
that are expecting the old darwin otool(1) behavior.

rdar://26828015

llvm-svn: 293547