[libclc] [libclc] Skip opt command if opt_flags is empty (PR #130882)

[Wenju He via cfe-commits]

wenju-he wrote:

> Another option would be to unconditionally have builtin.opt targets but if no flags are passed, just make them empty targets that rely only on builtin.link targets. 

Thanks for the suggestion. Done. Please review again.

https://github.com/llvm/llvm-project/pull/130882