[clangd] Collect tokens of main files when building the AST

Summary:
The first use of this is a code tweak to expand macro calls.
Will later be used to build syntax trees.

The memory overhead is small as we only store tokens of the main file.

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62956

llvm-svn: 363803