Remove the TargetTriple object that I added to the Driver recently. This

helped stage the refactoring of things a bit, but really isn't the right
place for it. The driver may be responsible for compilations with many
different targets. In those cases, having a target triple in the driver
is actively misleading because for many of those compilations that is
not actually the triple being targeted.

This moves the last remaining users of the Driver's target triple to
instead use the ToolChain's target triple. The toolchain has a single,
concrete target it operates over, making this a more stable and natural
home for it.

llvm-svn: 148942