[clangd] Minor cleanup

This patch silences few clang-tidy warnings, removes unwanted trailing
whitespace and enforces coding guidelines.

The functionality is not affected since the cleanup is rather straightforward,
all clangd tests are still green.

Reviewers: ioeric, ilya-biryukov

Reviewed By: ioeric

Subscribers: MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47471

llvm-svn: 333411