[clangd] A code action to swap branches of an if statement

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: llvm-commits, mgorny, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D56611

llvm-svn: 352796