Object, support both mach-o archive t.o.c file names

For historical reasons archives on mach-o have two possible names for the 
file containing the table of contents for the archive: "__.SYMDEF SORTED" 
and "__.SYMDEF".  But the libObject archive reader only supported the former.

This patch fixes llvm::object::Archive to support both names.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221747 91177308-0d34-0410-b5e6-96231b3b80d8