[libclc] [libclc] Update license headers (PR #132070)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

Note that this work is the result of asking the LLVM Foundation about the state of libclc licensing, and their checking with the Foundation's IP lawyer. I've CC'd @kbeyls for confirmation.

https://github.com/llvm/llvm-project/pull/132070