[clang] [amdgpu] Pass variadic arguments without splitting (PR #94083)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> @arsenm You're right about passing larger things indirectly. I'm intending to land this as-is, with the types inlined, as that unblocks #93362. I'm nervous that the extra pointer indirection will hit the same memory error that tweaking codegen in that patch hits (it's a similar sort of pattern to the top level argument passing) and wish to postpone that until there is a working baseline.

I do think we need to revisit some threshold at some point. We should use byref for anything that's most likely to hit the stack anyway 

https://github.com/llvm/llvm-project/pull/94083