SR-5289: Teach Mirror how to handle unowned/unmanaged references (#28368)
* SR-5289: Support reflecting weak, unowned, and unmanaged refs
This refactors how we handle reference ownership
when reflecting fields of struct and class objects.
There are now explicit paths for each type of reference
and some simple exhaustiveness checks to fail the build
if a new reference type is added in the future without
updating this logic.