[libclc] Make CMake messages better fit into LLVM (#86945)

The libclc project is currently only properly supported as an external
project. However, when trying to get it to also build in-tree, the CMake
configuration messages it outputs stand out amongst the rest of the LLVM
projects and sub-projects.

This commit makes all messages clear that they belong to the libclc
project, as well as turning them into 'STATUS' messages where
appropriate.