[clang] [llvm] [SPIR-V] Enable variadic function lowering for the SPIR-V target (PR #175076)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

This currently fails the `printf.ll` test because of the special handling SPIR-V does for `printf`. The easiest solution is to just ignore this if it's of the form that SPIR-V will mess with. Is it possible that we could mark those calls with a `nobuiltin` attribute or something? Let me know what the best course of action is here.

Variadics support as (hopefully) provided by this PR would let you just write a `printf` function, but I understand that SPIR-V has their own definition.

https://github.com/llvm/llvm-project/pull/175076