Today, we emit IL for struct/layout class marshaling similar to how we handle P/Invoke, Delegate, and COM interop marshalling stubs: We emit an IL stub always, put it in the IL stub cache, etc.

However, struct marshalling stubs have some weird aspects to them today:

1. We use one stub for all three operations: marshal managed->unmanaged, marshal unmanaged->managed, cleanup. We handle this today with a manual if-else decision tree (because our runtime IL emit tooling doesn't support switch) to jump to the right block.
2. Struct marshaling IL stubs are the only IL stubs that can be called directly in IL (from other struct marshaling IL stubs). As a result, we need to have more complicated handling for the IL stub's compile time state, just for struct marshaling stubs.
3. Struct marshaling stubs don't use a significant portion of the logic used with the rest of the marshaling stubs, so we've had to basically no-op those paths for struct marshaling.
4. The IL stub cache is too slow for common lookups from APIs like Marshal.StructureToPtr, so we had to add an additional faster caching layer in EEMarshalingData.

This PR moves struct marshaling to be handled via Transient IL, with the marshaling logic presented as the IL for each instantiation of System.StubHelpers.StructureMarshaler<T>.

I wanted to do the same for layout-class marshaling, but it's not possible to do so because there's no way to opt-out of generic sharing. However, I was able to structure it similarly by having layout class marshalling continue to generate IL stubs. In the layout-class case, the stubs are no longer cached alongside the rest of the interop stubs with all of that machinery. Layout-class stubs are cached on the managed side in statics in System.StubHelpers.LayoutClassMarshaler<T>, with a lock to ensure we don't double-generate the stubs.

This allows us to remove a decent amount of logic around struct marshaling IL stubs and how they interact with P/Invoke stubs. Additionally, it paves a path towards moving P/Invoke IL bodies to use transient IL bodies instead of IL stubs for non-shared cases.