[libclc] [libclc] Track dependencies through dependency files (PR #86965)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

> The build here seems to be trying to define clc as a language, which then results in needing to rely on language support magic like this. I think it would be better if this did what rocm-device-libs does and treat these as custom targets. I don't think it's particularly helpful to pretend like this is a normal host compiler detection situation.
> 
> https://github.com/ROCm/llvm-project/blob/efc7219da809b6bce7b1c92c1915a0eac68fb1f2/amd/device-libs/cmake/OCL.cmake#L136

Yes, I agree. It also makes building `libclc` in-tree a nuisance, because CMake needs to know the path to the "language compilers" at configure time, which leads to _fun_ like intuiting the final output path of `clang` and other tools and writing an empty string to those paths to ensure they exist on disk before you call `enable_language`.

So I agree that moving to custom targets would be beneficial for many reasons. I think overall it would be a simpler solution, not least because it's the more idiomatic way of doing CMake. I might look into that in the near future, but can't promise it.

https://github.com/llvm/llvm-project/pull/86965