Most PDB fields on disk are 32-bit but describe the file 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.
Differential Revision: https://reviews.llvm.org/D109923
