Workarounds for issue #104511

[kg]

Draft to check for failures on CI
Alternative to #111984

For two scenarios touched on in issue #104511, we can solve them through different paths in the same InitializeFieldDescs method:

Static field on a struct, of a generic struct type that indirectly references the type hosting the field, i.e.

struct X { static ImmutableArray<X> F; }

We have existing infrastructure for doing cycle-avoidant type loads in cases like this - tkTypeDefToAvoidIfPossible - so we just need to sense that we've encountered a scenario where it might be necessary. We can do this by checking whether the field is:

1. static
2. of struct type
3. of a generic struct type
   We already know 1 & 2, so I add a bit of signature decoding to determine 3. If all three conditions are met we use tkTypeDefToAvoidIfPossible to avoid loading ourselves when loading the field type. This feels like a decent compromise to fix this scenario and shouldn't impact the vast majority of fields in existing applications.

Static field on an interface, of a struct type that implements said interface, i.e.

interface I { static Y F; }
struct Y : I { }

tkTypeDefToAvoidIfPossible can't save us here and flowing it through into the place where interfaces are loaded would require some significant code changes, so I decided to see if I could narrowly target a workaround based on our existing handling of self-referential static fields.
This PR's approach is to look up the type of each static struct field on an interface, and if it's a typedef (not a typeref, which means it's in the current module and can form a cycle), we scan the interfaceimpl table for that typedef to see if we're in it. If we are, treat the field type as if it's a reference to us. This taps into the existing logic for handling self-referential static fields and seems to work, though I had to tweak some code related to enums.

The cost of doing this could get prohibitive for large numbers of static struct type fields on an interface, though I'm having trouble imagining a scenario that would require that, and there are some fairly obvious ways to optimize it.