[lld][PECOFF] Add WinLinkDriver and PECOFFTargetInfo.

Add WinLinkDriver and connect it to the existing COFF reader. Remaining
parts are still stubs, so while it can now read a COFF file, it still
cannot link or output PE/COFF files yet.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D865