Summary:
This is going to be needed to test e.g. diagnostics regeneration on
didSave where files changed on disk. Coordinating such changes is too
hard in lit tests.
Reviewers: kadircet
Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D77766
Sam McCall