[clang] [llvm] [transforms] Inline simple variadic functions (PR #81058)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

> High level question: Does this patch eliminate the variadic call edge, or, does it perform inlining on very special variadic function definitions? I thought the former but `isFunctionInlinable`, sufficiently confused me.

This patch will rewrite calls to a variadic function into calls to a function taking a va_list. Later patches expand that to cover the test of the cases. Note that calls to variadic function pointers cannot generally be rewritten without permitting ABI changes, which is what I plan to do for nvptx and amdgpu.

https://github.com/llvm/llvm-project/pull/81058