[libclc] [libclc] add --only-needed to llvm-link when INTERNALIZE flag is set (PR #130871)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

You're right, my mistake: even on this repo.

I was using `llvm-diff` and only checking the error code. It reports e.g., `function @_Z9__clc_minDv16_DhDh exists only in left module` but still returns `0` so I missed it.

This only affects the `builtin.link.*` bytecode libraries and not the final ones, presumably due to optimizations.

https://github.com/llvm/llvm-project/pull/130871