[clang] [llvm] [WIP][AMDGPU] Enable hostcall printf for OpenCL (PR #70932)

[Matt Arsenault via cfe-commits]

arsenm wrote:

For point 1, I would prefer to decouple the printf implementation choice from the language. I would expect consistent defaults regardless of the language. This also contradicts point 2? I thought the clang emitted path used hostcall, and the backend path did not.

As for expanding the printf implementation capabilities to cover vectors, that should be a separate patch from any behavior change 

https://github.com/llvm/llvm-project/pull/70932