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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Needs test. I also would assume you shouldn't have to reinvent language detection code

There's an existing function for getting the inputs, but it takes the `ToolChain` as input so it makes a circular dependency. The implementation doesn't seem to need _that_ much from the ToolChain so we could possibly factor it out?

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