Revert "[clang][modules] Use relative offsets for input files"

This reverts commit b9d78bdc. After this change, all libc++ `clang_modules_include.gen.py` tests started failing on Linux builders: https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-linux-arm64/b8771862804321535569/test-results