[CUDA] Add #pragma clang force_cuda_host_device_{begin,end} pragmas.

Summary:
These cause us to consider all functions in-between to be __host__
__device__.

You can nest these pragmas; you just can't have more 'end's than
'begin's.

Reviewers: rsmith

Subscribers: tra, jhen, cfe-commits

Differential Revision: https://reviews.llvm.org/D24975

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283677 91177308-0d34-0410-b5e6-96231b3b80d8