[Modules][PCH] Serialize #pragma pack

This patch serializes the state of #pragma pack. It preserves the state of the
pragma from a PCH/from modules in a file that uses that PCH/those modules.

rdar://21359084

Differential Revision: https://reviews.llvm.org/D31241

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299226 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 40be91d5ff7b8d289fdb0e9df4de724eb4d1a152)
12 files changed