[CUDA] Initialize our header search using the host triple.

Summary:
This used to work because system headers are found in a (somewhat)
predictable set of locations on Linux.  But this is not the case on
MacOS; without this change, we don't look in the right places for our
headers when doing device-side compilation on Mac.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26776

llvm-svn: 287286