ILLink should build type info after custom steps run for a type

[sbomer]

Context: #103987 (comment)

Custom steps rely on the ability to add methods to a type. This only works if the TypeMapInfo cache for that type is built after the custom step gets a chance to process the type, otherwise we miss interface methods that are implemented by the custom step.

As far as I can tell this was never guaranteed, and is heavily order-dependent. To make this guarantee for custom steps we might need to build the type info cache per type, instead of per assembly, and ensure it never gets built for a type before we call into the custom step for a type.

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