Changes

[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