Improve DWARFDebugFrame::parse to also handle __eh_frame.

LLVM MC has single methods which can handle the output of EH frame and DWARF CIE's and FDE's.

This code improves DWARFDebugFrame::parse to do the same for parsing.

This also allows llvm-objdump to support the --dwarf=frames option which objdump supports.  This
option dumps the .eh_frame section using the new code in DWARFDebugFrame::parse.

http://reviews.llvm.org/D15535

Reviewed by Rafael Espindola.

llvm-svn: 256008