[clang] [CUDA][HIP] add option -gpuinc (PR #140106)

[Artem Belevich via cfe-commits]

https://github.com/Artem-B commented:

Being able to override a flag is a good thing to have, IMO. There are builds where the owner of the leaf targets do not have much control over which options are set by the "default" compilation, so they need to rely on being able to override preceding options.

The fact that we didn't need it till now for this particular option, and that we may be able to work around this particular use case does not negate conceptual usefulness, but makes it a "nice to have" thing, rather than a "must fix that".

While we're at that, perhaps we should consider leaving those legacy-style flags as is, and  implement new-style overridable `--[no-]offload-{inc,lib}` ?

https://github.com/llvm/llvm-project/pull/140106