X86: Assembly files with .cfi_cfa_def shouldn't hit llvm_unreachable()

On darwin, when trying to create compact unwind info, a .cfi_cfa_def
directive would case an llvm_unreachable() to be hit. Back off when we
see this directive and generate the regular DWARF style eh_frame.

rdar://15406518

llvm-svn: 194285