[clangd] Enable clang-tidy by default.

Summary:
We have turned on the flag internally for a while, and we don't receive complains.
Should be good to turn it on now.

If the projects doesn't have .clang-tidy files, no clang-tidy check will
be run.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60560

llvm-svn: 358282