DON'T MERGE Brace for source imcompatible changes in Scala around Primitives.

**DO NOT MERGE**

Context: https://github.com/scala/scala/pull/4136

The constants and predicates about primitive codes are being moved from the scalaPrimitives object (through its parent trait ScalaPrimitives) to the ScalaPrimitives companion object.

This commit makes our codebase cross-source compatible with the compiler before and after these changes.
"makes a 'val ScalaPrimitives' available". What do you mean by only?