Can't use ref struct types

[degory]

It's not possible to use ref struct types reflected from assemblies built with C#. For example we want to be able to call methods on DefaultInterpolatedStringHandler to support string interpolation.

• Materialize correct types and symbols for reflected ref struct types so they can be used
• Prevent values of these types from being boxed (because they should be constrained to live on the stack)

A partial fix, sufficient to support internal use for string interpolation would be not to enforce the allocate only on stack constraint and warn on use about this unchecked constraint