[clangd] Limit the size of synthesized fix message

Summary: A temporary workaround until we figure out a better way to present fixes.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62372

llvm-svn: 361625