Deterministic builds with LLVM's GN build

Summary: Use the following args.gn.

use_relative_paths_in_debug_info = true

It is possible to produce [locally deterministic][1] builds of LLVM with the GN build. It requires some configuration though.

1. Make debug info use relative paths by setting use_relative_paths_in_debug_info = true in your args.gn file. With this set, current debuggers need minor configuration to keep working. See “Getting to local determinism” and “Getting debuggers to work well with locally deterministic builds” in the [deterministic builds][1] documentation for details.

1: http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html