Fix constrained call corner cases (#22464)

Fix constrained call corner cases (#22464)

Fixes #22423. I'm still unclear on how JitStress manages to get itself into that situation, but I was able to write a repro that triggers the `!pMDAfterConstraintResolution->IsInterface()` assert using regular IL, so we need to handle that either way. The repro for that is constrained3.il.

While figuring out the repro, I wrote a bunch of other test code and found another bug (constrained2), where we would box in a situations that doesn't require boxing (canonically ambiguous situation where there's a suitable default interface implementation and a valuetype implementation of the constrained method that does not requires boxing once we no longer deal with __Canon).