[llvm] [Offload][AMDGPU] Correctly handle variable implicit argument sizes (PR #142199)

[Shilei Tian via llvm-commits]

https://github.com/shiltian commented:

If the implicit argument is not full set, how can we guarantee the order and existence of those arguments?

> Additionally, we modify the alloc and free routines to allow
alloc(0) and free(nullptr) as these are mandated by the C standard
and allow us to easily handle cases where the user calls a kernel with
no arguments.

A separate PR pls.

https://github.com/llvm/llvm-project/pull/142199