[libclc] [llvm] [libclc] Support LLVM_ENABLE_RUNTIMES when building (PR #141574)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

I'm assuming the libc++ tests aren't failing due to this PR. They seem unstable, looking at jobs run for other PRs.

https://github.com/llvm/llvm-project/pull/141574