Issue Description:

The runtime technically has a distinction between ClassMetadata, which models the metadata of a pure Swift class, and AnyClassMetadata, which might be a Swift class or an Objective-C class. However, we use ClassMetadata in a lot of places where we really mean AnyClassMetadata. We're pretty good about checking for isTypeMetadata() before using any other members, but Clang is totally within its rights to hoist loads past those checks.