[clang] [libc] [Clang] Rename GPU intrinsic functions from `__gpu_` to `_gpu_` (PR #118674)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I'm not opposed, but it doesn't seem particularly well-motivated to me as I think the only real motivation is to work around a frustrating behavior in clangd. I think double underscores are more well understood by users to be reserved names.

https://github.com/llvm/llvm-project/pull/118674