Make -fuse-ld=lld work properly on Windows.

Using lld on Windows requires calling link-lld.exe instead of
lld.exe.  This patch puts this knowledge into clang so that when
using the GCC style clang driver, it can properly delegate to
lld.

Differential Revision: http://reviews.llvm.org/D6428
Reviewed by: Reid Kleckner, Rui Ueyama