[llvm] [LLVM] Add 'ExpandVariadicsPass' to LTO default pipeline (PR #100479)

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

The original idea was to run the optimise per-tu instead of on the whole module at lto time, but until there's a non-gpu target using it in production the distinction doesn't matter too much. It's presumably at lto time here in order to combine application code with libc which seems legitimate

https://github.com/llvm/llvm-project/pull/100479