[clang] [Clang][NFC] Introduce `gpulib` positive flag for `nogpulib` (PR #126567)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> I agree that we should be able to negate the option (and, perhaps --nocudainc, as well, while we're at that)
> 
> That said, perhaps we should reconsider how we implement it.
> 
> Originally `-nogpulib` was modeled after `-nostdlib`. So we've got a single-dash option, and no way to undo it.
> 
> Now that we do want to undo it, perhaps we should make it a normal double-dash offloading option. and alias `-nogpulib` to the negative variant of it? WDYT?

The inc flags aren't as problematic since they're only ever present on the frontend job. I'm ambivalent on changing the spelling, can do that without changing much.

https://github.com/llvm/llvm-project/pull/126567