Improve dotnet-pgo output and fix null type handles (#53494)

Improve dotnet-pgo output and fix null type handles (#53494)

* Improve dotnet-pgo output and fix null type handles

* When comparing .mibc files, find a unique name to print for both files
  by repeatedly appending another parent directory on it.

* When comparing .mibc files, print a comparison of type handle
  histogram data

* Fix a potential division by zero

* Print some info when producing .mibc that are based on samples/LBR
  data

* Fix .mibc read type handles with value 0 to actually return IsNull

* Use different devirt threshold for interfaces and classes

* Add ClassProfileFlags to PgoFormat.cs