[clangd] Patch PP directives to use stale preambles while building ASTs

Summary:
Depends on D79930.

This enables more accurate parsing of the AST, by making new macro
definitions in preamble section visible. This is handled by injecting
define directives into preamble patch.

This patch doesn't handle any location mappings yet, so features like go-to-def,
go-to-refs and hover might not work as expected. These will be addressed in a
follow-up patch.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79992