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

src/coreclr/tools/Common/Pgo/PgoFormat.cs

@@ -79,6 +79,14 @@ public struct PgoSchemaElem
                             ((InstrumentationKind & PgoInstrumentationKind.MarshalMask) == PgoInstrumentationKind.EightByte)));
     }
 
+    // Flags stored in 'Other' field of TypeHandleHistogram*Count entries.
+    [Flags]
+    public enum ClassProfileFlags : uint
+    {
+        IsInterface = 0x40000000,
+        IsClass = 0x80000000,
+    }
+
     public class PgoProcessor
     {
         private enum InstrumentationDataProcessingState

src/coreclr/tools/aot/ILCompiler.ReadyToRun/IBC/MIbcProfileParser.cs

@@ -35,7 +35,7 @@ TypeSystemEntityOrUnknown IPgoSchemaDataLoader<TypeSystemEntityOrUnknown>.TypeFr
                 try
                 {
                     if (token == 0)
-                        return new TypeSystemEntityOrUnknown(0);
+                        return new TypeSystemEntityOrUnknown(null);
                     if ((token & 0xFF000000) == 0)
                     {
                         // token type is 0, therefore it can't be a type