[clangd] Store ppdirective in Inclusion

Summary:
This will enable PreamblePatching proposed in D77392 craft a more
informed patch.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78235