[CUDA] Driver changes to support CUDA compilation on MacOS.

[CUDA] Driver changes to support CUDA compilation on MacOS.

Summary:
Compiling CUDA device code requires us to know the host toolchain,
because CUDA device-side compiles pull in e.g. host headers.

When we only supported Linux compilation, this worked because
CudaToolChain, which is responsible for device-side CUDA compilation,
inherited from the Linux toolchain.  But in order to support MacOS,
CudaToolChain needs to take a HostToolChain pointer.

Because a CUDA toolchain now requires a host TC, we no longer will
create a CUDA toolchain from Driver::getToolChain -- you have to go
through CreateOffloadingDeviceToolChains.  I am *pretty* sure this is
correct, and that previously any attempt to create a CUDA toolchain
through getToolChain() would eventually have resulted in us throwing
"error: unsupported use of NVPTX for host compilation".

In any case hacking getToolChain to create a CUDA+host toolchain would
be wrong, because a Driver can be reused for multiple compilations,
potentially with different host TCs, and getToolChain will cache the
result, causing us to potentially use a stale host TC.

So that's the main change in this patch.

In addition, we have to pull CudaInstallationDetector out of Generic_GCC
and into a top-level class.  It's now used by the Generic_GCC and MachO
toolchains.

Reviewers: tra

Subscribers: rryan, hfinkel, sfantao

Differential Revision: https://reviews.llvm.org/D26774

llvm-svn: 287285