Allow running back-deployment testing against libc++abi

Before this patch, we could only link against the back-deployment libc++abi
dylib. This patch allows linking against the just-built libc++abi, but
running against the back-deployment one -- just like we do for libc++.

Also, add XFAIL markup to flag expected errors.

GitOrigin-RevId: 4d79ef814aedb63e85903bb145dfe7ec143c6909
2 files changed
tree: 5fff0368f16d59762881b6bcdd4f5dd55ba1a4b3
  1. .clang-format
  2. CMakeLists.txt
  4. cmake/
  5. docs/
  6. include/
  7. src/
  8. test/