Support for primitive classes should be off by default in javac and enabled with -XDEnablePrimitiveClasses.

Restricted functionality should include:
- Declarations of primitive classes
- Recognition of primitive classes in the class reader
- '.ref' types
- '.default' expressions
- Optionally, internals that deal with primitive value types (or just leave it and it will never run)

Tests, including javac, Hotspot runtime, and Hotspot compiler, will need to be updated to use this flag.

Looking ahead, we will want to be prepared to entirely exclude code dealing with primitive classes if the Value Objects JEP is delivered independently.