[clang] [AMDGPU] Use the AMDGPUToolChain when targeting C/C++ directly (PR #99687)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

There's a test that's `.cl` that doesn't pass `-opencl` so probably can't rely on it.

https://github.com/llvm/llvm-project/pull/99687