[dslx] type info iteration order is nondeterministic

Right now to construct serialized type information for a module we traverse an unordered map, then sort by textual span start; however, when there's a tie on textual span start the order is still undetermined, need some criterion to make a total order.

Could traverse to number the AST in memory, but might be a little nicer if it could be done on the proto data instead of in-memory objects.