[clang] [llvm] [IPO] Implement common code for variadic lowering pass (PR #93974)

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

This is #93362 with the amdgpu part removed as debugging the reported amdgpu libc memory error is taking longer than one hoped and I'd like to unblock implementing other targets.

As it stands this is dead code other than the wasm tests which has the advantage that it definitely can't break anything. Developing on multiple branches is causing me a fair amount of overhead and confusion - if we can agree that a derivative of this looks reasonable, it'll let us review target specific implementation independently of one another and generally make the review cycle less redundant.

https://github.com/llvm/llvm-project/pull/93974