[clang] Persist Attr::IsPackExpansion into the PCH

Summary: Fixes https://github.com/clangd/clangd/issues/309

Subscribers: ilya-biryukov, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77194

(cherry picked from commit 8b3b7556)