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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223086 91177308-0d34-0410-b5e6-96231b3b80d8