The fix here is two-fold:
1) Teach SILGen that it cannot use the scalar casting paths for extended existentials
2) Teach the runtime casting entrypoint to unwrap as much metatype structure as possible
before arriving at a 'Self' type bound for the requirement checking paths.
The code here mirrors the destructuring check we're doing in remote mirrors.
rdar://95166916
Implement casting to and from extended existentials. This is done by slightly generalizing the conditional conformances checking infrastructure.
Unfortunately, casts for reference types and metatypes are unsound because IRGen is peepholing all non-opaque existential conversions with a helper. I’ll disable that in a follow-up.
rdar://92197049
