[clang] [clang][transformer] Add `merge` range-selector for selecting the merge of two ranges. (PR #169560)

[Yitzhak Mandelbaum via cfe-commits]

https://github.com/ymand requested changes to this pull request.

API addition looks, but some questions about implementation given the subtleties of ranges :(

https://github.com/llvm/llvm-project/pull/169560