The idea here is that we want to be able to create a reproducible hash of the PDB file before and use this as the timestamp field in the PDB and set the timestamp of the executable to match.
In order to do this, we need a way to defer emitting the signature until after everything else in the PDB file has been written. This way when we hash the PDB file, we hash it as if it had default values (e.g. all zeros) for the build id, and then update the build id field.
For now, this is NFC intended only to make the necessary refactoring to do this kind of deferred emission of the build id possible. No actual hashing of the PDB is done yet. We'll do that in a followup.