[CUDA] Add Sema::CUDADiagBuilder and Sema::CUDADiagIf{Device,Host}Code().

Summary:
Together these let you easily create diagnostics that

 - are never emitted for host code
 - are always emitted for __device__ and __global__ functions, and
 - are emitted for __host__ __device__ functions iff these functions are
   codegen'ed.

At the moment there are only three diagnostics that need this treatment,
but I have more to add, and it's not sustainable to write code for emitting
every such diagnostic twice, and from a special wrapper in SemaCUDA.cpp.

While we're at it, don't emit the function name in
err_cuda_device_exceptions: It's not necessary to print it, and making
this work in the new framework in the face of a null value for
dyn_cast<FunctionDecl>(CurContext) isn't worth the effort.

Reviewers: rnk

Subscribers: cfe-commits, tra

Differential Revision: https://reviews.llvm.org/D25139

llvm-svn: 284143