[libclc] [libclc] Fix commands in compile_to_bc are executed sequentially (PR #130755)

[Wenju He via cfe-commits]

wenju-he wrote:

> On my build this increases the number of targets `ninja -t targets` from 19K to 28K. That's building all targets. I'm not sure what else we could do about that and whether it's a problem.

yes, the number of targets increase significantly with this approach. But as far as I know, we don't have compilation correctness issue regarding the increase of the number of targets.
The only alternative approach is DEPENDS_EXPLICIT_ONLY. I can add DEPENDS_EXPLICIT_ONLY support to this PR if cmake version is 3.27 or higher.

https://github.com/llvm/llvm-project/pull/130755