Respect bound archs, even when they don't alter the toolchain.

Summary:
It's possible to BindArch without changing the toolchain at all.  For
example, armv7 and armv7s have exactly the same toolchain triple.

Therefore the code in the Driver that checks that we're not creating a
job for the same Action twice needs to consider (Action, Toolchain,
BoundArch) tuples.

Reviewers: tra

Subscribers: aemerson, echristo, beanz, cfe-commits

Differential Revision: http://reviews.llvm.org/D16250

llvm-svn: 257983