[CUDA] Improve target attribute checking for function templates.

* __host__ __device__ functions are no longer considered to be
  redeclarations of __host__ or __device__ functions. This prevents
  unintentional merging of target attributes across them.
* Function target attributes are not considered (and must match) during
  explicit instantiation and specialization of function templates.

Differential Revision: https://reviews.llvm.org/D25809

llvm-svn: 288962