[CUDA] Rename CheckCUDATarget to IsAllowedCUDACall. NFC

Summary:
I want to reuse "CheckCUDAFoo" in a later patch.  Also, I think
IsAllowedCUDACall gets the point across more clearly.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23238

llvm-svn: 278193