clang-cl: Support /link option and set target to win32

This adds support for the /link option, which forwards
subsequent arguments to the linker.

The test for this will only work when targetting win32.
Since that's the only target where clang-cl makes sense,
use that target by default.

Differential Revision: http://llvm-reviews.chandlerc.com/D1388

llvm-svn: 188331