[CUDA] Make CUDAInstallationDetector take the host triple in its constructor.

Summary:
Previously it was taking the true target triple, which is not really
what it needs: The location of the CUDA installation depends on the host
OS.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28319

llvm-svn: 291130