Fixes and Documentation for PGO

[nibanks]

Apparently PGO wasn't actually working before, for a couple of reasons. This fixes it for x64 and adds some initial documentation.

[ThadHouse]

Wouldn't we need a PDB per build specification? So we'd need all variations of x86, x64, mitls, schannel and any other build types we want running over PGO?

[nibanks]

Wouldn't we need a PDB per build specification? So we'd need all variations of x86, x64, mitls, schannel and any other build types we want running over PGO?

Yep. (Assuming I can get this to build). For now, I have it only dynamically picking up PGO if we specify a file for it. As for other TLS configurations, Schannel is the only one we care about, plus it still works if you use the .pgd from Schannel on miTLS builds.

[nibanks]

Ugg. I have no idea why it's not working now. It was building and running locally just fine. Then I pushed my PR. The PR build broke, then I tried a clean build locally and now it breaks locally too. I'm confused.