Java2Swift: Improve diagnostics when we suppress import due to a missing Java class import

When we run Java2Swift, we generally get a large number of warnings like this:

warning: Unable to translate 'java.util.Collection' method 'removeIf': Java class 'java.util.function.Predicate' has not been translated into Swift
warning: Unable to translate 'java.util.Collection' method 'parallelStream': Java class 'java.util.stream.Stream' has not been translated into Swift
warning: Unable to translate 'java.util.Collection' method 'forEach': Java class 'java.util.function.Consumer' has not been translated into Swift

The user experience isn't great for several reasons:

1. There are often a large number of warnings.
2. It's not obvious how to address those warnings.
3. It's not clear after-the-fact which APIs were dropped.

We should re-work how we report these kinds of issues. Specifically, I think we should report just one warning per target, and have that warning also produce the contents of a Java2Swift.config file that contains suggested mappings for those types, e.g.,

{
  "classes" : {
    "java.util.function.Consumer" : "Consumer",
    "java.util.function.Predicate" : "Predicate",
    "java.util.stream.Stream" : "Stream"
  }
}

Additionally, we could emit comments into the generated Swift sources indicating which APIs weren't mapped into Swift, and why, e.g.,

// Java API not available in Swift because java.util.Predicate has not been translated into Swift:
//   boolean removeIf(Predicate<? super E> filter);

Start by locating the Java2Swift diagnostics that report missing Java class imports, then inspect how Java2Swift.config mappings and generated Swift sources are handled. Done means consolidating warnings per target, emitting suggested mappings, and optionally adding comments for omitted APIs as described in the issue.