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

[Felix Weiglhofer via cfe-commits]

fweig 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.

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