[libclc] [libclc] Convert tabs to spaces in CMake (PR #85634)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

FWIW I don't think the build bots are actually building `libclc`.

Locally, at least, it's not sufficient to specify `LLVM_ENABLE_PROJECTS=libclc` - you _also_ need `LLVM_EXTERNAL_PROJECTS=libclc`. Even then, it doesn't build for me in-tree, because it assumes `CMAKE_SOURCE_DIR` is its own top-level directory, which it isn't as it's `llvm/`.

I had a patch locally to switch `CMAKE_SOURCE_DIR` to `PROJECT_SOURCE_DIR` which I think is sufficient to cover both build styles. Should I contribute this upstream?

Another problem is that all `libclc` targets are enabled by default, but the `spirv-mesa3d` and `spirv64-mesa3d` targets both need `llvm-spirv` which I doubt our buildbots have available. Should those be disabled by default, or should the buildbots disable them?

https://github.com/llvm/llvm-project/pull/85634