[clangd] Make signatureHelp work with stale preambles

Summary:
This is achieved by calculating newly added includes and implicitly
parsing them as if they were part of the main file.

This also gets rid of the need for consistent preamble reads.

Reviewers: sammccall

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, mgrang, arphaman, jfb, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77392