Implemement default interface methods constrained call corner case

[MichalStrehovsky]

Implement the corner case where interface method implemented on a struct is getting constrained-called and the target of the constrained call is compile-time time ambiguous between a method on the struct and a default-implemented method. CoreCLR throws an NotSupportedException, Mono will box when necessary. Implement whatever is more convenient.

See disabled tests referencing this issue.

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