[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

llvm-svn: 299226
diff --git a/clang/lib/Sema/SemaAttr.cpp b/clang/lib/Sema/SemaAttr.cpp
index 34744cb..3ffaab2 100644
--- a/clang/lib/Sema/SemaAttr.cpp
+++ b/clang/lib/Sema/SemaAttr.cpp
@@ -215,6 +215,7 @@
                                        ValueType Value) {
   if (Action == PSK_Reset) {
     CurrentValue = DefaultValue;
+    CurrentPragmaLocation = PragmaLocation;
     return;
   }
   if (Action & PSK_Push)