IRGen: metadata for noncopyable exist. metatype
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
