[clangd] Replace getLangOpts().isHeaderFile usage with isHeaderFile helper.

Summary:
The helper is more correct to detect header file, this would fix our
issues caused by false positive before.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: merge_guards_bot, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70299