[cdac] Make GetObjectData for arrays use canonical method table when getting the element type

[elinor-fung]

When getting the element type for an array, cDAC was just getting the CorElementType of the element type handle. However, there are cases where that does not match the element type on the class for the array's canonical method table - for example, arrays of multi-dimensional or non-zero-based arrays. This changes the cDAC to use the canonical method table when getting the element type - matching the DAC (which uses MethodTable::GetArrayElementType).

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[elinor-fung]

@davidwrighton It came as a surprise to me that MethodTable::GetArrayElementType would return something different from MethodTable::GetArrayElementTypeHandle().GetSignatureCorElementType(). I don't know the implications (if any) of those two not matching - I'm not sure if this just expected or if it could be problematic in some way?

I see how we get there by using the object[] method table:

runtime/src/coreclr/vm/array.cpp

Lines 226 to 237 in 7cd266e

	// Arrays of reference types all share the same EEClass.
	//
	// We can't share nested SZARRAYs because they have different
	// numbers of constructors.
	//
	// Unfortunately, we cannot share more because of it would affect user visible System.RuntimeMethodHandle behavior
	if (CorTypeInfo::IsObjRef(elemType) && elemType != ELEMENT_TYPE_SZARRAY && pElemMT != g_pObjectClass)
	{
	// This is loading the canonical version of the array so we can override
	OVERRIDE_TYPE_LOAD_LEVEL_LIMIT(CLASS_LOADED);
	pCanonMT = ClassLoader::LoadArrayTypeThrowing(TypeHandle(g_pObjectClass), arrayKind, Rank).AsMethodTable();
	}

The only case I could figure out where this would happen is an array of a multi-dimensional or non-zero-based array - for example, ushort[][,]. The CorElementType of the element type handle (ushort[,]) is ELEMENT_TYPE_ARRAY in that case, which doesn't match ELEMENT_TYPE_CLASS of object[] .

[jkotas]

This is historic artifact of how arrays were represented in the past and fragile NGen.

It may be best to delete ArrayClass::m_ElementType field, GetArrayElementType methods, and replace their calls with GetArrayElementTypeHandle().GetSignatureCorElementType/GetVerifierCorElementType as appropriate.

[elinor-fung]

Ah, thanks. I'll look at doing that.