[clangd] NFC: Simplify code, enforce LLVM Coding Standards

For consistency, functional-style code pieces are replaced with their
simple counterparts to improve readability.

Also, file headers are fixed to comply with LLVM Coding Standards.

`static` member of anonymous namespace is not marked `static` anymore,
because it is redundant.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D52466

llvm-svn: 342974