[libTooling] Add an `EditGenerator` that applies a rule throughout a bound node.

The new combinator, `rewriteDescendants`, applies a rewrite rule to all
descendants of a specified bound node.  That rewrite rule can refer to nodes
bound by the parent, both in the matcher and in the edits.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D84409