[clangd] Handle additional includes while parsing ASTs

Summary:
Enables building ASTs with stale preambles by handling additional preamble
includes. Sets the correct location information for those imaginary includes so
that features like gotodef/documentlink keeps functioning propoerly.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77644