Vary Windows toolchain selection by -fuse-ld

Summary:
This change allows binutils to be used for linking with MSVC. Currently, when
using an MSVC target and `-fuse-ld=bfd`, the driver produces an invalid linker
invocation.

Reviewers: rnk, compnerd

Reviewed By: compnerd

Subscribers: smeenai, cfe-commits

Differential Revision: https://reviews.llvm.org/D39509

llvm-svn: 317511