Add Metadata to Transformer tooling

This change adds a Metadata field to ASTEdit, Edit, and AtomicChange so that
edits can have associated metadata and that metadata can be constructed with
Transformer-based RewriteRules. Metadata is ignored when applying edits to
source, but other consumers of AtomicChange can use this metadata to direct how
they want to consume each edit.

Reviewed By: ymandel, gribozavr2

Differential Revision: https://reviews.llvm.org/D82226