PR13189: va_list broken with precompiled headers

For some targets a structure named __va_list_tag is built to help define
the __builtin_va_list type.  However, __va_list_tag was not being treated as a
predefined type thus causing problems when serializing the AST.  This commit
fixes that oversight by adding the necessary support to treat __va_list_tag
as a predefined type.

llvm-svn: 159508
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 8b354f3..b118685 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -4461,6 +4461,9 @@
       T = Context.ARCUnbridgedCastTy;
       break;
 
+    case PREDEF_TYPE_VA_LIST_TAG:
+      T = Context.getVaListTagType();
+      break;
     }
 
     assert(!T.isNull() && "Unknown predefined type");