track portable pdb woes

[ctaggart]

Some end users have been have perceived issues with the portable pdb format or its tooling.

fscheck/FsCheck#378

devlooped/moq#428
Issue with Moq.pdb in version 4.7.99: "Symbol indexes could not be retrieved"

[ctaggart]

Looks like moq ran into lack of support in TFS. It will be in TFS 2018.
devlooped/moq#447
https://developercommunity.visualstudio.com/content/problem/33776/indexed-source-information-could-not-be-retrieved-1.html

[stakx]

@ctaggart, seeing that you referenced the Moq issue, I'd just like to mention one more thing I noticed (but haven't yet found the time to analyze in-depth): dotnet sourcelink test (v2.2.1) might have problems with portable PDBs, too (it complains about an invalid COR2.0 header sometimes), but this could also be a usage error. I could run a few more experiments if you're interested.

If you want to expand the above list, you can find more TFS-related issues with Sendgrid and Noda Time referenced in the Moq issue.