[libTooling] Change Tranformer's consumer to take multiple changes

Previously, Transformer would invoke the consumer once per file modified per
match, in addition to any errors encountered. The consumer is not aware of which
AtomicChanges come from any particular match. It is unclear which sets of edits
may be related or whether an error invalidates any previously emitted changes.

Modify the signature of the consumer to accept a set of changes. This keeps
related changes (i.e. all edits from a single match) together, and clarifies
that errors don't produce partial changes.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D119745