IRGen: metadata for noncopyable exist. metatype

[kavon]

Missed a case when determining the strategy to use for obtaining the metadata for a noncopyable existential metatype. Without this patch, trying to use a metatype like any ~Copyable.Type can cause a crash at runtime when setting a deployment target that predates Swift 6.0, due to that runtime not knowing how to demangle inverses.

resolves rdar://134280902

[kavon]

@swift-ci smoke test

[kavon]

@swift-ci please test macOS platform

[rjmccall]

I'm a little surprised this is represented as a ProtocolCompositionType; should we just be looking for existential types in general?

I assume the only reason this is specific to existential metatypes is that we don't currently allow non-copyable existential types in Swift?

[kavon]

@rjmccall We use to map InverseTypeRepr to a full-fledged InverseType, but we ended up changing the representation so that ~Copyable is just an empty ProtocolCompositionType with an "inverse Copyable bit" set.

So, this change in GenReflection covers any type we want to get-by-mangled-name that contains an inverse. I don't know off-hand what uses of existentials will trigger getTypeRef but I can try to add test coverage for those too, since we do have noncopyable existentials.

[kavon]

@swift-ci smoke test

[rjmccall]

Alright, that sounds fine to me.