Fix PGO trace data collection in the presence of collectible assemblies

[davidwrighton]

We didn't handle unknown type handles in the data. Currently this can only happen for collectible assemblies.

Fixes #67877

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[jakobbotsch]

Doesn't this mean we will overestimate the likelihood of non-dynamic classes when static PGO is used? There are comments indicating this is the intention of unknown type handles in the probe helpers:

runtime/src/coreclr/vm/jithelpers.cpp

Lines 5355 to 5362 in 84b664a

	// If the object class is collectible, record an unknown typehandle.
	// We do this instead of recording NULL so that we won't over-estimate
	// the likelihood of known type handles.
	//
	if (pMT->GetLoaderAllocator()->IsCollectible())
	{
	pMT = (MethodTable*)DEFAULT_UNKNOWN_TYPEHANDLE;
	}

[jakobbotsch]

Ah, I realize this is only used to figure out which types we need to emit type detail ETW events for, so disregard the above comment.