Create a runtime mapping from Java canonical class names to their equivalent Swift types

[DougGregor]

When we have a Swift type that wraps a Java class, it has a macro annotation like this:

@JavaClass("java.lang.Object")
struct JavaObject { ... }

Given the type JavaObject on the Swift side, we can query the AnyJavaObject conformance to get the canonical Java class name. However, we have no way to go in the opposite direction, from a Java class name like java.lang.Object to JavaObject. The translation manifest has this information for use in the tool, but at runtime there is no way say "find the most specific Swift type for this Java object".

We should consider some kind of automatically-populated runtime registry to make this possible.