[clangd] Make FixIt message be consistent with the clang-tidy diagnostic message.

Summary:
We strip the "[clang-tidy-check]" suffix from the clang-tidy diagnostics, we
should be consistent with the message in FixIt (strip the suffix as well).

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63926

llvm-svn: 364731