[llvm] [PDB] Correctly read/write the named stream map's niMac value (PR #225729)

[Hans Wennborg via llvm-commits]

zmodem wrote:

> You could probably do "llvm-pdbutil explain" with `-offset=0x110C2` to actually cover the `niMac` in `InjectedSource.pdb`?

Hmm, but it is covered. It's just that the NSM is now considered 4 bytes longer, since the niMac value is considered to be part of it, so the expectation needs a small tweak.

https://github.com/llvm/llvm-project/pull/225729