[clangd] Add helper for collecting #include directives in file.

Summary: Separate unit tests for the new function will be added in followup patch which will further refactor Headers.h

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D46675

llvm-svn: 332237