[compiler-rt] [compiler-rt] Use installed libc++(abi) for tests instead of build tree (PR #115077)

[Louis Dionne via llvm-commits]

https://github.com/ldionne commented:

I'm not very familiar with the compiler-rt build, but I love this in principle.

We shouldn't be making assumptions about the internal state of a project's build outside of that project itself. The "public" interface of libc++ is the installation step so that's the only thing we should be using from compiler-rt.

Thanks for this cleanup!

https://github.com/llvm/llvm-project/pull/115077