[lld][PECOFF] Read relocation entries.

Summary:
COFFReference class is defined to represent relocation information for
COFFDefinedAtom, as ELFReference for ELFDefinedAtom. ReaderCOFF can now
read relocation entries and create COFFReferences accordingly.

I need to make WriterPECOFF to handle the relocation references created by
the reader, but this patch is already big, so I think it's probably better
to get it reviewed now.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D976