Most PDB fields are 32-bit, but in terms of MSF blocks, which are 4 kiB by default.
So PDB files can be a bit larger than 4 GiB, and much larger if you create them with a block size > 4 kiB.
This is a first (necessary, but by far not not sufficient) step towards supporting such PDB files.
Now we don't truncate in-memory file offsets (which are in terms of bytes, not in terms of blocks).
No effective behavior change. lld-link will still error out if it were to produce PDBs > 4 GiB.