[clang] [llvm] [CodeGen][LLVM] Make the `va_list` related intrinsics generic. (PR #85460)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

Is LLVM IR using the old vaarg intrinsics still consumable?

Should we update AutoUpgrade.cpp to auto upgrade the old vaarg intrinsics and add a test for them?

https://github.com/llvm/llvm-project/pull/85460