Publish symbols using Microsoft.SymbolUploader.Build.Task package

[dagood]

The Microsoft.SymbolUploader.Build.Task package on the BuildTools feed is the new way to index and publish symbols to the symbol server. It should be used instead of the current combination of BuildTools' GetAllSymbolFilesToPublish and the VSTS symbol publish build step.

Example implementation in Core-Setup: dotnet/core-setup#3465

[mikem8361]

I haven't been able to test this (all the changes are done) because the coreclr pipe build hasn't passed in a week or so.