Type equivalence support in Crossgen2 (#87899)

Type equivalence support in Crossgen2 (#87899)

We currently have a small behavior hole on Windows where if a binary uses TypeEquivalence, it is technically possible to get crossgen2 to generate invalid code.

This fixes that by adding support for type equivalence to the managed type system, and adjusting crossgen2 to use it. This is complicated by the detail that this may generate some type references which cannot be referenced from an R2R file, so additional error checking was needed (this additional error checking is what fixes #67855).

However, while basic support for type equivalence has been added to the type system, support for actually performing type equivalent interface resolution has not been, so devirtualization of type equivalent interface calls is disabled. At this time, I do not expect to see a customer need for implementing that fairly complicated feature

Fixes #67855