[clang] opencl: Ensure printf symbol is not mangled. (PR #150210)

[Sven van Haastregt via cfe-commits]

svenvh wrote:

> @svenvh, could this fix be backported to the 20.x branch? It might be a while until our OpenCL runtime (PoCL) supports newer Clang versions.

Correct me if I'm wrong, but I believe `release/20.x` is now closed and no more 20.x releases are being planned.
We can backport it to 21.x if that helps (but that won't help 20.x based environments).

https://github.com/llvm/llvm-project/pull/150210