[clang] [llvm] [WIP] Expand variadic functions in IR (PR #89007)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

Drive by fixes to some of Matt's comments. Caught a missing line in a .def file for NVPTX through luck due to the enum rename which means the Other/new-pm-thinlto-postlink-samplepgo-defaults.ll style tests need to be patched again - leaving that for now as I want to check whether the passes run in the order of the PassRegistry.def (in which case expand-variadics should probably happen before always-inline) and the failing test is a reminder to myself of that

https://github.com/llvm/llvm-project/pull/89007