Microsoft.AspNetCore.App symbols missing in 3.1 source-build

[dagood]

Opened from #1564:

In our source-build .NET Core 3.1 we include pdb files for Microsoft.NETCore.App dlls.

Also, we don't seem to have pdb files for Microsoft.AspNetCore.App, so we may be missing debug info for ASP.NET framework?

[MichaelSimons]

This was resolved with #3547.