Summary:
The current handling of `libclc` is to use custom clang jobs to compile
source files. This is the way we used to compile GPU libraries, but we
have largely moved away from this. The eventual goal is to simple be
able to use `clang` like a normal project. One barrier to this is the
lack of language support for OpenCL in CMake.
This PR uses CMake's custom language support to define a minimal OpenCL
language, largely just a wrapper around `clang -x cl`. This does
nothing, just enables the support. Future PRs will need to untangle the
mess of dependencies.
The 'link+opt' steps that we now do should be able to simply be done as
a custom `llvm-link` and `opt` job on the library, which isn't ideal but
it still better than the current state.
