swift234

MetadataSource Improvements: Builder; Parent, Impossible kinds

Create a builder divorced from the ReflectionContext so that
MetadataSources can be created in other contexts, such as emitting
private heap metadata during IRGen, where we'll have to record the
layout of captures and how to get metadata for generic arguments in
order to construct typerefs of the captures, etc.

Add Parent, Metadata capture, and Impossible metadata sources.