[libclc] [llvm] [libclc] Refactor build system to allow in-tree builds (PR #87622)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

CC @mgorny -  I was unsure whether you are actively using and interested `libclc` or you were just looking after build bots or some other reason. I've successfully built the SPIR-V libraries out-of-tree but another pair of eyes can't hurt.

I was wondering - if we `libclc` building in-tree, would there be interest in building the SPIR-V targets with the LLVM SPIRV backend, rather than the external `llvm-spirv` tool?

https://github.com/llvm/llvm-project/pull/87622