[clangd] Collect macros in the preamble region of the main file

Summary:
- store all macro references in the ParsedAST;
- unify the two variants of CollectMainFileMacros;

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, mgrang, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67496

llvm-svn: 372725