Revert "[libc++abi] Stop copying headers to the build directory" (#115232)

Reverts llvm/llvm-project#115086

2-stage sanitizer build is not happy:
https://lab.llvm.org/buildbot/#/builders/25/builds/3915

GitOrigin-RevId: d08772b1512f630240d8b7feaab749e659d3fce8
2 files changed