[clangd] Improve the output of rename tests where there are failures.

Summary:
Previously, we match ranges, which is hard to spot the difference.
Now, we diff the code after rename against the expected result, it
produces much nicer output.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69890