Update J2CLItableMerging to consider custom descriptors

[mason-lgtm]

Update J2CLItableMerging to consider types whose vtables are custom descriptors

[tlively]

You might want to reserve field 0 in the descriptor. Once we have JS interop, the JS prototype will (almost certainly) have to be stored in field 0.

[tlively]

It seems we should have a similar check before looking up the name of field 0. Also, we can avoid double lookup by doing this:

Suggested change

	if (!typeNameInfo.fieldNames.count(1) ||
	!typeNameInfo.fieldNames[1].equals("itable")) {
	if (auto it = typeNameInfo.fieldNames.find(1);
	it == typeNameInfo.fieldNames.end() || it->second.equals("itable")) {

[tlively]

If these would be empty, then we'll just continue to the next loop iteration anyway, so I don't see a reason to make them optional.

Suggested change

	std::optional<HeapType> vtabletype;
	std::optional<HeapType> itabletype;
	HeapType vtabletype;
	HeapType itabletype;

[tlively]

A HeapType is represented as a single number, so no need to take a reference to it unless you're going to mutate it.

Suggested change

	HeapType& javaClass = parent.structInfoByITableType[curr->type.getHeapType()]->javaClass;
	HeapType javaClass = parent.structInfoByITableType[curr->type.getHeapType()]->javaClass;