[clangd] Expose SelectionTree to code tweaks, and use it for swap if branches.

Summary:
This reduces the per-check implementation burden and redundant work.
It also makes checks range-aware by default (treating the commonAncestor
as if it were a point selection should be good baseline behavior).

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57570

llvm-svn: 352876