[clang] [AMDGPU] Treat printf as builtin for OpenCL (PR #72554)

[Vikram Hegde via cfe-commits]

vikramRH wrote:

> Any tests? Can you explain why it's not sufficient to do this lowering in the AMDGPU pass?

I intended these changes to be part of https://github.com/llvm/llvm-project/pull/72556, but it seemed too many changes at one place, so I extracted this part out for ease of review. This cannot be merged standalone and has to be with 72556 , the tests are also part of that patch (This really should have been a stack of patches :( ). 

Also for AMDGPU pass, I plan to remove that altogether and handle all printf lowering at one place during clang Codegen. since we now use a compiler option to switch between different implementations, This makes a lot more sense I feel.

https://github.com/llvm/llvm-project/pull/72554