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

[Alexander Richardson via llvm-commits]

arichardson wrote:

We should not be using the headers from the build dir, this will be removed in #115380 . It sounds to me that the problem is DESTDIR being set while building since that affects the destination of the local libc++ installation. I'll see if I can make the ExternalProject_add() call ignore DESTDIR.

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