[libTooling] Add assorted `EditGenerator` combinators.

Summary:
This patch adds various combinators that help in constructing `EditGenerator`s:
   * `noEdits`
   * `ifBound`, specialized to `ASTEdit`
   * `flatten` and `flattenVector` which allow for easy construction from a set
     of sub edits.
   * `shrinkTo`, which generates edits to shrink a given range to another that
     it encloses.

Reviewers: asoffer, gribozavr2

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84310