[clang] [CUDA][HIP] Fix deduction guide (PR #69366)

[Tom Honermann via cfe-commits]

tahonermann wrote:

For what it is worth, the described behavior sounds right to me from a design perspective. The fact that it matches `nvcc` behavior is a very good hint that it is the desired behavior as well. I haven't reviewed the code changes, but as long as they implement what is described, I'd give this a thumbs up.

https://github.com/llvm/llvm-project/pull/69366