[clangd] Fix modernize-loop-convert "multiple diag in flight" crash.

Summary:
this maybe not ideal, but it is trivial and does fix the crash.

Fixes https://github.com/clangd/clangd/issues/156.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78715

(cherry picked from commit a466e4be)