Revert "[libc++] Stop copying headers to the build directory (#115380)" This reverts commit 428c8767ae997b0f726c0b40160ea8172551babf. Breaks sanitizer build: https://lab.llvm.org/buildbot/#/builders/51/builds/9056 GitOrigin-RevId: 576b53801fc3d721602ae0d8377af9950f356000